TOPIC: Overlapping control management

Hello there at DT register !
I desabled "overlapping control" in Global configuration (because of the Login problem) but I would like to enable this control at 'Event' level (because I have 4 events in a category that occure at the same period of time).
Unfortunately I don't understand the difference between your 2 parameters :
- "Override Global Overlapping check"
- "Exclude event from Overlap check"
They look identical for me.
Which one I have to enable for being able to make an Overlap check (when Global configuration says "NO" ? (with no interference with "login" - even if I want 'User auto-creation' on these events).
Any solutions ? Thanks in anticipation for your help.
Note: My site is under construction and off-line (reason why I don't send a 'ticket' but a forum post ...