Ok Fred,
there are two types of readings in the DROs, machine coords that you do not normally see and work offsets that you normally do see.
The Machine coords are set when you reference the machine, often they will be zero at the home position but you can offset them if it is more appropriate for your machine. For example is your home switches are 100mm away from the end of travel you can set a Home Off (offset) value on the homing and limits page and so when you home, instead of the machine coords DRO getting set to zero it will be set to 100. This tells Mach that the extent of the axis is 100mm away from your home position.
Now the work offset, they are, as I said, the readings you normally see and can be anything as it depends on what you set them to. For example if you have a Home Off value of 100 set and home the machine you could set the Work Offset to 100 also by clicking the DRO, typing 100 and pressing the enter key. That will mean that each time you home the machine the work offset DRO will show 100.
If you leave the Home Off value at zero you can still do the same as above, when you home the machine the machine coords will be set zero but you can still enter any value into the DRO and that will be your work offset value, so you home, the machine coords get set to zero (you dont see unless you are viewing Machine Coords) you can then enter the value you wish in the DRO. Each time you home the DRO will show the value you entered.
Hood