Out Of URLs
Every so often, someone will get a message to the effect of "Empty URL" or "Out of URL's".
According to the SL wiki, all parcels will have a URL "pool" that is basically 1 capable URL per prim allotted to the parcel.
So if you have a parcel that supports 100 prims, it can also support 100 URLs.
The Linden wiki states that you can combine the URL pool over multiple parcels. Casper has seen indications that the parcels can only share the URL pool if they are actually joined (making a larger parcel).
If you are trying to use rental units, vendors and/or dropboxes on a parcel and are getting constant "empty/out of URLs" messages, then you need to either boot the sim (in case it's a temporary problem) or look at the number of prims you have available on the parcel, and compare that to the number of scripted items that need URLs.
Region/parcel "Object bonus" only applies to prims, not URL's.
Thus, it is the specific parcel that the scripted items are on that matters for the URL load.