It's unlimited in that you can request time off at any time with no balance or speed limit. The first order justification is that it saves some money from having to pay out unused time off balances at the end of the year / tenure or have people burst time off at random synchronous periods. However the second order effect is that the "how much time off is too much time off" is never defined and is a moving target. "If I take four weeks this year and things get tight next year will I show up on a filtered list for layoffs?"

It's uncomfortable for employees but employers tout how comfortable it is.

No written rules is fascism.