This referendum enables two approval voting protocol improvements that are a prerequisite to scale up the number of parachain validators to 500.
Once enacted, the validators will start using the new format for approval voting assignments and also approve multiple candidates under a single singature. The impact is the reduction of network bandwidth and CPU usage of the approval voting protocol.
High level tracking issue for scaling the validator set: https://github.com/paritytech/roadmap/issues/26
Relevant node side changes (released Q4 2023):