Areas are the physical subdivision of a venue (as a physical configuration). They can be numbered (with seats) or not numbered.
Sector is an attribute for a Area (a simple String, like Floor), it has no functional impact.
Blocks are an optional sub division of seated Areas. Blocks are typically not used in small venues.
The Area (and optionaly block) define a seat, together with row and seat number: areaCode, (blockCode), rowNumber, seatNumber
The Block (or the Area if the block is not defined) is key in the optimal selection. The system will first try to seat in the best ranked Blocks. It's also possible to configure the system so that it does not return seats that would be in different Blocks.