This is the name of the telescope focus, using the standard ESO archive notation . The name begins with an abbreviation for the telescope's governing organization. The next part of the name is the aperture in decimeters (preceded by letters A, B, etc. if there are multiple telescopes with similar apertures). The name ends with the suffix P for prime focus, A for Cassegrain focus, C for coudé, and Nx for Nasmyth foci.
Thus, for example, the Cassegrain focus of the ESO 1.5-meter telescope is designated ESO15A. But this telescope has an asymmetrical mounting that is difficult to switch from one side of the pier to the other. In this case, it is necessary to operate on one side only during the night; this is indicated by appending the letter E or W to show which side of the pier the telescope is actually used on. So, for the 1.5-m telescope used east of the pier, the designation of the Cassegrain focus is ESO15AE.