Server-wide event handlers are absolutely identical to user-specific ones with the 3 following remarks:
1.They are executed always, regardless of who is the connected user (thus they're run for all users)
2.They are always executed after the user-specific equivalents
3.The "before sending server ID to client" event handler is executed only if it's defined server-wide (because there is no such event at user-level).
You configure server-wide event handlers from the Event Handlers page of the Event Handling section of the Configuration Manager.
When 2 or more scripts are set to be executed upon occurrence of the same event (e.g. 2 scripts are both set to be executed "after a file is uploaded") they will be run in order of Priority, lower to higher. This means, for instance, that a script with priority set to 10 will be run before a script with priority set to 20.