As for:
In previous versions (here pictured with 7.21) when you create a pool with prefix-length with a wider prefix length (here 60)
and later request a narrower subnet from the pool (here /64), the prefixes taken from the pool all have the wider prefix length:
I've assigned two /64 to two interfaces (containers and containers-2) and each took a /60 chunk from the pool.
In 7.23, if we define a pool with prefix-length=60 setting:
We can later request smaller prefix chunks from that pool. In this example, a /62 sub-pool was created from the parent pool, as well as 4 address assignments, each with /64:
We can now see under Used Prefixes that we've not taken out 5 /60 chunks out of the pool, but one /62 and four /64.
With 7.23, combined with the from-pool-policy=without-acquire, it's possible for users whose greedy ISPs only allocate a single /64 prefix, to both use that prefix for SLAAC (assign to one interface with from-pool-policy=without-acquire) as well as to use the same /64 pool for a DHCPv6 server instance that assigns /128 addresses.