The what shows what versions of object modules were used to construct a file. For example:
# what /bin/ls
/bin/ls:
Copyright (c) 1989, 1993, 1994
termcap.c 8.1 (Berkeley) 6/4/93
strftime.c 7.38
strftime.c 5.4 (Berkeley) 3/14/89
getbsize.c 8.1 (Berkeley) 6/4/93
localtime.c 7.57
asctime.c 7.7
rec_seq.c 8.3 (Berkeley) 7/14/94