Every ANI file consists of an array, used for making in-game animation.
MSTA-0.ANI is an array of frame index. Then the others frames are given in the MFRA-0.ANI array. Single elements of frames are given by MELE-0.ANI.
All ANI files have different formats - elements of the arrays they store are different in every file. They are probably similar to those used in Syndicate. The ANI files from Syndicate are reworked and discussed in libSyndicate.
Un-reworked ANI files:
DATA\MELE-0.ANI DATA\MFRA-0.ANI DATA\MSTA-0.ANI DATA\NFRA-0.ANI DATA\NSTA-0.ANI DATA\TMAP.ANI