Think of it more like this
(command "dim" "hor" pt1 pt2 pt3 "" "exit")
pt1 is left pick point
pt2 is right pick point
pt3 is dim line pick point
all are calulated in some way or picked
(list 90 -18 0.0)) move the point +90 in X direction -18 in Y direction Z no change
You could do a after making dim make a choice, LL UL UR LR for the 4 directions. For me I use a library program that makes a dcl on the fly.