This referendum updates the collator sets and configurations for all Polkadot system parachains. For Asset Hub, it adds five new "invulnerables" to the existing two, and changes the number of candidates from six to seven.
For Collectives and Bridge Hub both, it initializes the candidate deposit to 1,000 DOT (same as Asset Hub), opens up seven candidate positions, and also sets seven invulnerables per chain.
The seven invulnerables per chain were chosen by a group of node operators who have self-organized around providing collation services and demonstrated the ability to run nodes on other system parachains.
For more details on the motivation of adding so many invulnerables, see Fellowship RFC-7.
Threshold