Hello Kelly,
Sorry for that. Those words are terms in the travel and hospitality industry. Room allotment means the number of rooms alloted to a tour operator by a hotel, with a cut-off date. So, if this agency receive an booking (within the maximum number of rooms that are alloted, and prior to the cut-off date), they can confirm to their clients before sending the request to the hotel.
Serial bookings means series of hotel reservations booked by a tour operator. The number of rooms are estimated, as they just want to block the rooms first. Then, they will try to sell their packages to the clients. Before the cut-off date, they have to reconfirm to hotel the exact number of rooms required.
I'm not sure if my explanation is good (as English is my second language!). Sorry for that.