Generally, it means the flight is fairly full. Normally, 60-70% of the seats can be assigned in advance. Once those seats are assigned, then the flight becomes "airport restricted".
I have also seen it in a situation where there was an equipment substitution planned and no seats could be assigned in advance.
You will also see it on code share flights where one airline's inventory is gone, but the other's is completely available. I had it happen on a QF
/AA flight operated by QF
but I was on an AA
flight number in C class. All it took was a phone call by the res agent to have the seat I wanted released.