valid - A block of Conditions to check if a Leader can have this Mandate. (Leader scope)
parameters - Actual functionality is unclear.
on_term_started - A block of Effects to be executed when the term of a Leader of this Mandate is started. (Leader scope)
on_term_ended - A block of Effects to be executed when the term of a Leader of this Mandate is ended. (Leader scope)
Whenever a leader becomes the ruler or a candidate of an Empire with an Authority that has use_mandates = yes, a random Mandate is selected for the leader. It will change the Mandate next time it become a candidate again.
Unlike Agendas, the Mandates don't have some sort of weight. All capable Mandates will appear in an equal chance.
In-game texts claim that a leader failed to fulfill its Mandate will become less likely to win the next election, but this has never been implemented since Stellaris v1.0 and until now.
All the principles the voters will follow is that they will vote for the candidates in the same faction as them or how many times was the candidate supported by the player. There is no way to modify how likely a Pop will support a candidate.
There is a Condition to check the Mandate: has_mandate = <mandate key>, but there are no Effects to change the Mandate.