Hi, so this is the basic break up. When HDB says quota, what they mean is that if you qualify for quota, you have that priority no matter what. Eg if you have PPS (50% quota) and total of 10 SoBF units up for taking, 5 will only be given to PPS. Exception is if there is less applications for each flat, the the quota will go to general category (or if people with this quota choose not to pick a flat).
So the simplified break up is as follows:
1st timers
50% for PPS
30% for MCPS
15% for rest
2nd timers
5% for 2nd timers
So there are other schemes and also specific rules how each scheme affects each SoBF grouping, eg. Less than 20 flats no 2nd timer quota, etc. Just make sure you read the terms so you know what affects you in which way.