On a mill the tool offsets are a way of telling the control the difference in length from one tool to the next.
When you set tool 1 as the datum tool, all others are set in offset as the difference in length from tool 1.
E.g.
1 Insert tool 1 into the spindle.
2 Move down until the top of the work is touched. Zero out Z axis.
3 Do work for that tool.
4 Load next tool, move down until it touches the top face of the work.
5 The value in the Z axis DRO is the offset for this tool . Enter this value in the offset you want to use for this tool.
6 In the NC code for this tool you should have 'G43' and a H followed by the offset number to use, this will use the offset on this tool.
Repeat for next tool.
Is this what you wanted?, if not just say.
Graham.