Hi Max,

> When MoI joins separated parts it changes ID's of the faces. So I can't find
> which faces are new created.

Yeah that's just basically how ID values work - they are basically tied to a specific version of an object. When an object is edited in any way, the new objects generated from that then have their own new ID values as well.

Maybe instead of setting up a style to track it you could assign object names to the faces you want to track, I think they should persist across the separate as well.

- Michael