To purchase season tickets for most venues and their sections, you are required to purchase a type of team/venue product, sometimes known as a PSL (Permanent Seat Licenses). These products can be viewed as an asset, they have different names and terms depending on the team or venue they are purchased from. Some of the other names these go by are Stadium Builder Licenses (SBLs), Memberships, Stadium Seat Licenses (SSLs), Charter Seat Licenses (CSL), Seat Options (SO) etc.
Sometimes, the team Ticket Office is sold out. If you want to become a season ticket holder, you can request to be put on a waitlist or visit a marketplace to purchase seats from a current season ticket holder, this way you you have rights to any current and future seasons.