Ian,
You have a dedicated lathe DRO hence the L at the end. The later SD6's can be configured to anything, however all is not lost.
It's basically a reader and the only difference between lathe and mill is that lath *can* be programed for diameter reading.
If however its programmed for radius reading then it works exactly the same as a mill.
When you switch in it does a self check test, and flashes up lathe, the far right digit in the X display tells you the resolution. If its 5 then it's radius mode and will work as a mill display if it's 10 then it's programmed for diameter giving double the reading.
To change this if needed, switch off then switch on and as it's booting press the decimal point button and this takes you into program mode.
First line says X RESLN and will display 10
Press 5 then ENT then the down arrow. to exit press ENT then the decimal key again.