By the time you have captain, you will have one extra tag storage, but you don't need captain rank to get it. You just need to beat all 50 assaults and Nyzul. I got the extra tag when I completed my 51st assault. I still haven't gotten captain yet.
Supposedly, you need 50 assaults, Nyzul, and AMs complete for captain. I'm missing the AMs; ie. I do not have captain rank, but I do have an extra tag. I've been working on assaults and postponing the AM fights. Considering how easy the AMs have been so far compared to the assaults, it's not surprising that folks are associating the tag with the captain rank. A couple of days has pushed me up to AM35. It would have been faster if it wasn't for the "JP midnight" speed-bumps. It took 3 months to clear assaults.
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