You're not giving it a distance from the source [after all, a dimension line can often be a different distance from the two definition points, so which should it use?].  Rather, you are giving it a location for the dimension line.  If you just type in a number by itself, that location will be at the distance you call for in the direction of the cursor location from the last-picked point in the process, which latter will vary depending on the type of Dimension, whether you gave it two points to dimension between or used the select-object option, etc.

I find in experimenting that it doesn't even honor ORTHO mode relative to the last point in locating that position, which disappointed me.  But if you're doing horizontal/vertical Dimensions and by giving it two points to dimension between, you can get the location you want relative to your second point by typing in @300,0 or @0,300 [sometimes with minus sign on the 300] or @300<0 [or other angle as appropriate].