The three volume Oxford Dictionary of Byzantium has a half page article on this. Basically it was a high dignitary with different duties in different ages. Up until the tenth century, however, it was a common title for commanders of military themes. In later times it was (also?) a high court official. There were bearded and eunuch divisions and at times the title was even given to foreign princes. It is all rather confusing because we are talking an 1,100 year empire and stuff evolves. The Oxford even gives the insignia. I can copy and send you the article if you can't get it via interlibrary loan at an academic library. I hesitate to post it because of copyright issues. But if you want the article drop me an address to send it to at my mailing address Apuleius Books, Box 234, Garrison, NY 10524.

My area of expertise lies elsewhere, but deferentially, I am not aware of this as a rank. This is likely a granted title and not actually part of the command structure.

If you could shed more light on where you came across this, it might help.

The Protomandator was the head of all the messengers, and the Protomanglavites were a class of palace guards. I'm not familiar with protosphatarius.