This is an oddball feature from turbo assembler I always liked. An example will illustrate this better than I would:
dx.b 'string 1'
dx.b 'string 2'
This will deposit the 'string 1' bytes, then deposit 58 (64-8) spaces (ascii 32), then deposit 'string 2' etc.
Quite handy if you want to make an fixed size table but are too bored to do the filling by hand.
The second parameter in dxset is optional and defaults to 0.
If the first parameter in dxset is smaller than the line deposited (for example dxset 2 / dx.b 'aaa') then that's an error.
Finally, turbo assembler limits dc.X to strings but maybe we could extend this to anything?