From what I've seen this whole pathway (including a seniority number in some cases) from a turbo prop section/subsidiary of an airline group to the mainline or jet fleet has been a total cluster fcuk in most cases.
The guys and girls who have progressed the quickest from the turbo prop fleet through the system onto mainline or jet fleet have been those who have gone outside of the system and left the group altogether and come back to the jet fleet as external applicants. Sad and perverse but true.
Those that show loyalty are the ones that end up being screwed.
Even those with a seniority number at mainline still miss out as they are not being paid the salary their seniority would have granted them at their "new fleet".
While the idea of a progression pathway and or seniority number is a very noble one and gives the sense of being looked after and feeling of security of progression it actually gives an outcome that is the reverse to its intention, especially in this demographic environment.
There is no incentive for airline management to move guys and girls up the ladder when their time comes as it hurts the airlines too much. Perhaps if there were a financial penalty involved they may be more interested. From the airlines perspective it's much better financially, to poach pilots from from outside at the risk of upsetting a few of their own pilots. That way you can transfer your financial pain to someone else.
If there was no defined/agreed pathway then airline management would not have the safety net of knowing they have a group of pilots that have a vesting interest in staying around. Pilots would be more likely to jump ship if they weren't getting a fair deal which in turn would encourage airlines to be somewhat more proactive in looking after the pilots within their group.