Best In Slot.
It's just a general term used in MMO's to let you know which is literally the best option for a specific piece of equipment.
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Check my gear in lodestone, I have 481 accuracy with it. My crafted gears are headgear, chest, and ring. AF2 so far are pants, gloves, then getting boots next. I'm not following the "what's best to spend your myth" path. I like upgrading my stuff but I also wanna look good lol. When I get my bard's sandals I lose a lot of accuracy but I'll be able to compensate by crafting another ring(they're not unique) or using Deviled Eggs meal. I can easily make both coz I'm a lvl 50 LW and Culinarian and even if I weren't, those deviled eggs aren't expensive in my server.
Hope this helps even just a bit.