I was going to buy the "costume" slot but when I found out that it's actually an "uniform" slot I decided against it. To me, naming it a "costume" slot implies that one can use the slot for either uniforms or off duty clothes and the description itself makes no mention of it being restricted.

"This one is for the fashionistas! If you want to flaunt your style without having to run to the tailor all the time, purchase additional costume slots! Note: Purchase of this option adds two (2) costume slots to all characters on an account. Both Federation and Klingon characters may use this option."

Well a purchased costume slot can be switched to off-duty use.
You just have to tailor a uniform with it first.
The next time you tailor the slot you will be able to choose the
off-duty type on the 3rd (clothes) page.

So first I need to pay for the uni then exit the tailor and then go back in and the off duty option will be unlocked? If that's the case then I'm cool w/ this, even though it's a bit of a money sink.

I purchased the 2 costume slots and used 1 to create an off duty formal dress for my female Ferengi last night. Today I went back into the tailor to make some alterations BUT I have no option for any kind for off duty clothes and all I see are uniform options. I went ahead and reported this because it's a real annoying bug.

Ticket number #965,934

Yes, please make the bought ones multi-purpose! I want a second off-duty costume

Worked...thx for the info.