Individual parcel owners within a sim can use temporary prims according to the following formula:
temp_prim_limit = prim_limit - current_prim_count + min( 0.5 * current_prim_count + 400, 1000)
These are allocated over and above the normal prim limit for the parcel, and it is implicit in the calculation that they should not count against the sim prim limits.
However, these temporary prims are counting against the 15,000 sim limit, so that parcel owners using temporary prims can prohibit other parcel owners on the sim from using their normal prim allocation! This is a serious problem for many parcel owners at this time.
Reproduction:
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1. Fill a region with 14900 prims
2. Rez a 255 prim temporary object
3. Try to create a prim
4. Receive "Server full" message.