This service is intended to make it easy to create subscriber only Minecraft™ servers.
'Regular old' Minecraft aka The Java edition, not the Bedrock codebase version.

This service does not periodically sync a list of accounts to your server, instead it allows the server side mod to make a request and instantly see if a player is authorized to join. This makes for a 0 minute delay between subscribing and being able to join.

Useful links:

Supported services