Yes - it’s more like call-ahead-seating at some local restaurants - it gets you in line for the next table for your size party. They used to call it “Priority Seating” - not sure why they changed it as that makes it so much clearer.
As far as being late, I don’t believe there is anything official (and I’d be surprised if there was as that would encourage people to be late) in terms of a grace period. You are supposed to check in 5 minutes before your time (I could swear I’d seen 15 officially, but looking at the T&C I’m only seeing 5) - but there is nothing in the T&C about a grace period.
Do they allow people to “be late”? Most places likely will, BUT it will depend on the crowd levels, how busy the restaurant is - and sometimes human attitude and choice. If you are accidentally late (and I mean you left plenty of time to get there but something such as a transportation breakdown (which they can check) prevented you from being on time), and are apologetic they are likely to accommodate you if they can. If you go up to the check-in desk with an entitled attitude, they may not be as willing to accommodate you.
Just keep in mind that when you are late and they put you in line, you are then contributing to the backlog of ADRs and people waiting for tables that you have probably experienced elsewhere.
Bottom line - the polite thing to do is not purposely plan to be late. Accidents and delays happen, and they understand that. But when it becomes clear people are taking a lah-di-dah attitude about it and choosing to be late on purpose, I suspect we may see the “grace period” - official or unofficial - go away.