Thank you for your prompt answer !
This function was announced before AC21 launching and AFAIK it's not been activated.
Follows the extract from Help Center :
(https://helpcenter.graphisoft.com/guide ... servation/
Auto-Reservation of Elements
Note: This feature will be introduced in a later release of ARCHICAD.
•Whenever possible, existing elements are reserved by you automatically after you select them and begin a modification. In case it is not possible – because one or more selected elements are reserved by someone else, or if you must first Receive unsent changes – then a warning will appear telling you what to do.
•The Auto-reserve function is checked by default at Options > Work Environment > Data Safety & Integrity. If Auto-reserve should slow down your work, you will be prompted to uncheck this feature. You can then Reserve elements as needed, by clicking the Reserve button in Teamwork Palette or from context menus.
So it seems it's not already ok...