This may be an XY question! They occur when the questioner has a solution in mind 'Y', and asks about that rather than the root problem, which is 'X'. Even though my systems analysis training covered it in depth because it's incredibly hard to get users to explain what they really want a computer for, I still do it myself.
I think Simon's real question is: 'On a Taig Lathe, what's the difference between a front tool post and a rear tool-post? Or, 'Can a Taig rear tool-post be used as a front tool-post?'
Judging by the Rotagrip device at the end of DiogenesII's link, the same block of metal is used for both purposes on a Taig. Only the position changes:
![](https://www.rotagriponline.com/datasheets/Peatol/images/1170%20(2).jpg)
So, we're all off with the fairies, debating Gibraltar tool-posts, parting off, and industrial techniques, whilst it looks suspiciously as if all Simon needs is a basic Taig fitting costing £7.80.
As the Taig is a tiny lathe, it's likely to struggle with parting-off, in which case don't do it! Instead, saw the item, and face it off. But this is for later, when Simon has got the lathe going, and starts cutting metal.
Dave