Complex sentence: The 18 teams that hold UCI ProTeam status are obliged to participate in all races: the organisers of each race can invite other teams that hold UCI Pro-Continental status, or an ad-hoc national selection, to compete.
Split into simpler sentences: The 18 teams that hold UCI ProTeam status are obliged to participate in all races. The organisers of each race can additionally invite other teams that hold UCI Pro-Continental status, or an ad-hoc national selection, to compete.