If it helps, at my daughter's school, they differentiate "tank tops" from sleeveless, by saying if the strap is more than 3 fingers wide, it's sleeveless and acceptable. I'm not saying that Disney goes by my school rules, but surely they came up with that definition some where - it may be some universal definition.
I just wish that they were consistent and specific. If it's no shorts, then no shorts... If its no tank tops, then define it, since that is sort of a generic name that we give to shirts without sleeves.

The entitlement of those who think that they can get away with whatever they want because they "paid for this vacation" is what really gets me mentally judging.
We enjoy dressing up for dinner!
I did hear about a new rule about open backed shoes, but I haven't read it myself (don't worry I will)... If so, I'm clearly going to have to buy some shoes, because I think all my summer shoes are open backed!