AT&T 6300 Programmer's Manual page 98

Disk File Handling
4-14
There is no restriction on the depth of a tree
(the length of the longest path from root to leaf)
except in the number of allocation units
available. The root directory will have a fixed
maximum number of entries, 64 or 112 files for
a diskette. The maximum number of files in a
hard disk root directory depends on the size of
the MS-DOS partition on the disk.
Other sub-directories can also be accessed via
the root directory, and these in turn can branch
off to further files and sub-directories. The only
limit is the amount of available space on the
disk.
Old (pre 2.0) disks will appear to MS-DOS 2.0 as
having only a root directory with files in it and
no sub-directories whatever.