Hi iv'e been looking at that video link you sent me and it is very informative and helpfull so thank you for that. As far as the DRO is concerned if i set the depth of say 1mm on the DRO screen yes it doe's show the -1mm. Here is an image of the GCS in use with a Coke sign which i'm working on.
Click image for larger version.  Name: gcs test 1.png  Views: 151  Size: 158.6 KB  ID: 29508G-CODE CNC BACKUP.txt This on is the setting of my cnc as followed per instructions of that tutorial video you showed me.
I hope this helps , i'm not sure on all the termanoligybut ill get there.
Thaqnk you freind Colin