Can someone explain why they use the Z instead of making the 75 one unit with the international portion being senior?

I’ll let others answer your specific question but wanted to point out that “senior” isn’t always the same as international here at brown.

Case in point, back when we still flew 727s our most senior airplane and domicile was SDF 727 - which was all domestic. So lots of out & backs, many nights spent in your own bed and yet the very same pay rate as the folks who’re flying 747s flying around the globe.
With a single pay scale seniority is more about type of trips and QoL than pay rates. (we do have a small international pay override but it’s nothing I’d switch airplanes for)

I wouldn't say it's not commutable. MIA base has a lot of day-flying schedules. If you can bid the 10 to 13 day "deep south" trips, you are golden.

It has to do with keeping people current and qualified for the international flying. It was often difficult on the dc8 which did both.

Those trips are untouchable for junior guys, definitely wouldn’t count on it for years. It takes bidding inside 30% in seat to hold those trips consistently, other than the occasional VTO or open time trade/pick up.

Gotcha, how much domestic does the Z do? How long to hold it?

There’s plenty of Zomestic...

It’s interesting how they split the base but they both fly domestic.

You would be able to hold Z on the first system bid most likely...that being said you probably wouldn’t want to. You would go from having pretty decent seniority to having zero control of your schedule for probably at least two years. I’ve been a here a year and can’t hold anything, I just get whatever is not the flavor of the month. I have had pretty good luck with the trip board though.

The "zmestic" goes pretty junior