Not exactly keeping inbound under the current balance, but max capacity. For example, if you initially fund Phoenix (or any LN channel) for 100,000 sats, that is the maximum capacity you’ll ever be able to have in that channel.

If you spend the channel down to 90,000 sats, you now have a maximum inbound capacity of 10,000 sats. If someone were to send you 15,000 sats, Phoenix would open a 2nd channel to be able to receive the 5000 sats your 1st channel cannot handle.