Luke Jordan and many other men like him ended up in a Development Battalion because they were considered temporarily unfit for any other unit.

To be classified as fit for duty, the ideal would be to have a man classified as Class A: Free from disease, physically fit (able to perform a day's labor, walk five miles, have normal hearing and eye-sight) and more or less be able to function and serve as a normal servicemen whether in a combat unit or in a logistical or other types of support units. Before we dive deeper into what made a person unfit and thus eligible for a development battalion, we need to ask ourselves the obvious question.

What exactly was a Development Battalion?

At the time of the publishing of Major E.B. Johns' book Camp Travis and its part in the world war (1918/1919), he states that there were 51 of these battalions spread across the United States to improve, to develop, men to be fit for service. Men who had already been assigned to units but were found to be incapable of serving in a normal unit where removed and placed in a development battalion if they hadn't already been placed there immediately.

What made a man unfit to serve and made them eligible for a development battalion? There were many different categories. For example, you could be an individual who simply can't adjust to military life. You could also have a curable injury, disability or disease (such as a venereal disease) which keeps you from performing to your full extent but isn't serious enough to warrant an immediate discharge. You could be illiterate or be an immigrant who can't write or speak English that well, in which case you were put in an educational section where you would be taught how to write and speak proper English so as to function well in a military context. You also had men who were conscientious objectors being sent to these battalions as well as "alien enemies" - nationals of the Central Powers who were drafted into the American Army.

We get an example of how it could look like for a man to end up in a development battalion. From Johns:

The man came in the first draft from East Texas, and in his physical examination it was found that he had a bad leg and was unfit for service. He was sent home but not discharged. With the next draft, back came the man. Still nothing had been done to his leg, and still he was unfit for service. Again he was sent home, but he still could not be discharged. Third draft, back comes the man to Camp Travis, bad leg worse and less fit for service than ever. This time, however, he was sent to the development battalion. There he was turned over to the physicians and surgeon, who made a thorough and comprehensive study of his case. An operation was decided upon. This had now been performed and the man is recovering rapidly in the Base Hospital. After three weeks of convalescence, these operative cases are brought back to the development battalion where they are given the particular physical training suited to their needs.

When put in a development battalion, you would have been evaluated and placed in a section that suited your personal issue. If you were illiterate, you'd be placed in an educational section where, as mentioned before, you'd be instructed in the English language. If you were physically unfit, like the East Texas man previously mentioned, you'd be placed with other men in your own situation and given physical training suited to your particular needs. Specific help was given to those who required it, as shown above in the case of the East Texas man or as in the case of the 16th Battalion at Camp Travis that was made up of men with orthopedic issues. Those men who had flat feet were actually given special shoes to solve some of their problems as well as physical training.

Johns again:

Various exercises have been devised to strengthening the weak muscles. A man walks up an incline on his tip-toes and down on his heels; he walks on a little trough contrivance that makes his feet turn in, and in this way and others the trouble with his feet is corrected.

If against all odds you failed to develop or improve your condition, whether it be mentally or physically, you would be discharged.

Now, since I'm not familiar with Luke Jordan, I wouldn't be able to tell you why he ended up in a development battalion or what his particular battalion consisted of. Every camp had their own battalions and they were all numbered the same way (so there could be several 7th Development Battalions, for example). Either way, I hope this helps you understand Luke Jordan's war time service.

Sources:

The Medical Department of the United States Army in the World War, Volume XIII. Section IV, Development Battalions (1927).

Camp Travis and its part in the world war by Major E.B. Johns (1918/1919).

Another good reference (I was going to answer, but Bernadito beat me to it) is: "Report of the Surgeon General to the Secretary of War", 1919, Volume II, especially pages 1031 and following. The development battalion appears to have been sort of a combination of a remedial physical training battalion and a "medical holding" unit and a general training unit for service members who were not currently fit for full military service. On 9 May 1918, the War Department issued General Orders # 45, which directed the establishment of such units at every large camp or cantonment. The whole function of them was to rehabilitate less than fully-physically fit soldiers, or to recommend assignments for them in duties which did not require full physical fitness. It is noted that "about 15" camps developed such units which were felt to be very effective (more definitive numbers may be found in subsequent Surgeon General Reports, but I don't have time right now to do the research. In these units, troops were rapidly classified and either promptly discharged or given courses in physical development (under the supervision of medical officers) with the goal of enabling them to perform at least limited military duties. Some of these units had special companies for soldiers with VD, where they were kept until treated or diagnosed as non-treatable. About 225,000 soldiers were assigned to these units, of which 34% were VD, 13 % Orthopedic, 2% mental conditions, 5% heart complaints, and 24% "miscellaneous physical conditions". Of interest is that 22% were classified as "non-English speaking, illiterates, morally unfit, conscientious objectors, draft evaders, and enemy aliens"--- so it wasn't used solely as a medical rehab unit, as originally envisioned, but as a holding/training unit to evaluate and improve military capabilities of soldiers with many deficiencies. Only 18% were returned to full military duty (Class A), 20% to limited overseas duty, 19% to limited stateside service, and 36,000 were discharged from the Army. Just under a 1000 deserted, and 1356 died. I have not found any references to the battalions being numbered, so have not been able to look up the 7th Development Battalion. I suspect that information on this may be found in the book series "The Medical Department of the U.S. Army in the World War", but I don't have that at hand right now. I hope that helps a bit.