Exactly. There has to be a Capt SLI slot available in the fleet and, due to PWA language, you have to have at least 500 hours as a PIC of a Delta plane before they will hire you into a Capt SLI slot. As a result, you see FO SLIs leave the department to upgrade and get their 500 hours of Delta PIC and then come back as a Capt SLI if/when there are Capt SLI slots to fill.

It gets more interesting if you get hired as a Captain SLI (PWA requires 40% of all SLIs to be Captain SLIs) but you can't hold Captain on the airplane - more likely for a WB SLI who, perhaps, flew as a NB captain and had the requisite Delta PIC time and maybe flew as an FO on the WB fleet previously. (e.g., 330B who goes to 320A and then gets hired as a 330 Capt SLI but can't hold 330 Captain on the line). Then you get paid at the highest Capt category you can hold (7ERA?) but fly the line during your fly month(s) as an FO at your seniority while getting paid 7ERA pay. Those pilots have a pretty sweet deal until someone junior to them converts as a Captain on the fleet (at any base). Then they are required to upgrade to Captain on their fleet. That is why you will sometimes see a Captain in a category who is noticeably more junior than the next most junior Captain (also applies to project pilots).