It is different for each organization, appears to be random. It is populated even when no limit on full licenses are assigned. Sometimes it is more than the total full licenses assigned.
Available licenses are calculated based on the assigned licenses and by subtracting the number of assigned licenses and reserved licenses for the current organization path.
If an organization does not have a limit, LFDS looks at its parent organization up the path until it finds a limit.
For example, say you have an organization A with a sub-organization B and say you have 100 licenses in the master license. When you look at the limit for A, the number will 100 minus the number of already assigned licenses, but including any licenses assigned to users in A. So if you assigned 5 users with no organization, the max available for A will be 95. If you assign one named user in A, the number will still be 95.
For B, you will also get the same behavior, unless you set a limit for its parent. If you set A's limit to, say, 30, then B will have at most 30 licenses available. If you assign a named user license to a user in A, the number of licenses available to B will drop by one.
Then for all organizations under root it should always be just the total number of remaining licenses in the system? Yet each organization has a different remaining licenses count.
If I have 100 licenses and 91 licenses assigned to users (regardless of their membership) then the available licenses would be 9.
Assigning licenses to an organization will decrease available licenses for all sibling organizations (because they're not in their available pool anymore).
Also licenses assigned to users in that organization won't decrement the available licenses.
So if Organization A has 95 available licenses, and then you assign 45 licenses to users in A, A will still have 95 available licenses. If you then add sibling Organization A2 and assign it 30 licenses, A will have 65 licenses available and A2 will have 50 licenses available.
So in your screenshot, that organization (and its children) can only currently use 2 licenses, but you could potentially assign 7 more.