A league sequence is a connection of two leagues that occur across different seasons, but take place for the same
(1) Sport
(2) Location
(3) Day of the Week
Therefore, to sequence a league is to connect a league to either its past version or its future version (assuming that version has been created). Once these leagues are connected (sequenced) through the admin, other powerful features such as the "Missing Teams Report" and the "Play it Again" feature are available to help "farm" and sell future leagues and teams.
Not all leagues have a perfect sequence - For example an indoor volleyball league might switch to be an outdoor league in the summer, or take a break based on seasonality. So it is up to each individual league to decide how lax they want to be with sequences. The idea is not to over do sequences and try and connect every league. Rather connecting the leagues that have consistency season after season.