All filesystem related commands (crfs/rmfs..) are managing /etc/filesystems file automatically, so usually /etc/filesystems should not be edited manually. (However it is possible with vi.) There is another method which updates /etc/filesystems and this is the command ‘imfs’, which is not documented, but exists with AIX since the 90s.

If /etc/filesystems file has been removed or some parts have been deleted/overwritten accidentally imfs can create (or remove) filesystem related stanzas.

imfs -x <vgname> <–remove all filesystems related to the given vg from /etc/filesystems
imfs -xl <lvname> <–remove a filesystem (logical volume) stanza from /etc/filesystems

imfs <vgname> <–it recreates all filesystems in /etc/filesystems which are in the given vg
imfs -l <lvname> <–it recreates one filesystem in /etc/filesystem

(I think this command is using the vg and lv data on the disk and in the odm to recreate info in /etc/filesystems.)