Besides the standard FITS keywords defined in the FITS documents, it is possible to use other, non-standard keywords. Such non-standard keywords have no global definition and may be interpreted differently by different FITS readers.
A number of non-standard keywords has been used by ESO e.g.
for definition of frames acquired at the La Silla observatory.
They are given in Table 
and should not be used
with other definitions.
Table: Name translation between special non-standard FITS keywords and
MIDAS descriptors.
Due to the freedom to use non-standard keywords it is important to know their excact definitions when transfering data between different systems.