Characters can only be in one thread per timeslot.
Locations are generally expansive enough to allow multiple concurrent threads to exist at the same time with exceptions for some smaller interior locales in which case do try to keep to one ongoing thread per timeslot in that situation.
In locations which allow for multiple threads please also label if the thread is open or closed to uninvited participants. For locations of a size to allow only one thread logically it is always considered open unless it is a private dwelling or other hidden place.
Example of subject line:
For Want of a Thread (LN, The Maze, Open)