Book a workspace in Outlook

Great feature - can see a lot of useful scenarios (for example, where a pool of equipment like laptops exist, but we don't want to create an individual resource for each piece of equipment).

However PeterForster is completely correct - you shouldn't be using CustomAttribute11 for this purpose.

Here's the promise you made to customers about these attributes since they first existed, stating that you won't use them in Exchange attributes (emphasis mine):

'The custom attributes available to Exchange Server are labeled in Active Directory as ms-Exch-Extension-Attribute1 through ms-Exch-Extension-Attribute15. In the Exchange Management Shell, the corresponding parameters are CustomAttribute1 through CustomAttribute15. These attributes aren't used by any Exchange components. They can be used to store Active Directory data without having to extend the Active Directory schema.'

You've even got the custom attributes 16-45 that we were told not to use, since they might be used in future versions:

'ms-Exch-Extension-Attribute-16 to ms-Exch-Extension-Attribute-45 are present in Active Directory, but aren't available in the Exchange admin center (EAC) or the Exchange Management Shell. Don't use non-Exchange tools to edit these attributes because they might be used for future Exchange features.'

Who on earth decided to re-purpose an attribute that for years we have been promised would not be used by Exchange components, instead of creating a new attribute (or at least using one of the ones that we were told might be used by Exchange features)?

https://docs.microsoft.com/en-us/Exchange/recipients/mailbox-custom-attributes?view=exchserver-2019