01In plain English
A seat is one user. With per-seat pricing the plan price is multiplied by the number of people who need access. Some tools charge only for people who edit and let viewers in free, which changes the total a lot.
02Why it matters when you are choosing
A tool that looks cheap for one person can be the expensive choice for a team of ten. Comparing per-seat tools with flat-rate ones only works once you multiply by your real headcount.
03What to check
Ask the vendor
Check whether viewers or guests need a paid seat, whether there is a minimum number of seats, and whether you can move a seat to someone else.