Don't know if the following diagram helps, or actually makes it more confusing!
View attachment 655825
Basically if we had a Frappe team, we could enter up to three A riders (not that we have 3 but...) and as many B's or below as we wanted. If it was a Latte team, then it would be three B's max, with the rest being C's or D's. And that's assuming you have 5-8 riders in total - if you only start with 3 or 4, then there's a further restriction.
I've been in a TTT with B's before (early ZRL) back when I was an inexperienced borderline C/D. Then I rarely lasted half the course distance and that was with me going flat out while the B's just cruised on the front. As now a mid-C with a decent history in TTTs, I would feel able to contribute to a Latte team, but wouldn't stand a chance in a Frappe team - I'd be more of a drag on my teammates than I'd be useful in increasing the draft.