This will be a short answer as the question is related to a minuscule technical detail.

The XX+XX designation present in the description of the German planes from the Second World War is simply the side marking of an individual airplane that in the case of military planes were indicating actual tactical unit and specific plane belonging to that unit. So, they essentially mean the same thing as the unit designation (in the quoted case, 6th squadron [Staffel] of the 3rd Transport Wing [Transportgeschwader]) and allowed quick identification of a specific plane when airborne. They usually followed the XX+XX format, although either doublet could have been substituted by a single sign (letter or number) or missing completely, especially in the case of fighters that also utilised various icons to denote wing and squadron. In general, marking consisted of two doublets consisting of letters or numbers, positioned to the left and right of the Luftwaffe cross, represented by a '+' sign in the descriptions and located on the side of the fuselage, close to the tail.

It should be noted that this was typical of the military aircrafts, as German civilian planes followed the international guidelines that called for the single- or double-letter code of the country, followed by a dash and the three of four sign long registration code, e.g. D-TMBT in case of German aircraft operated by e.g. Lufthansa.