Enabling users to install apps from sources outside of official or trusted sources presents a potential threat of introducing harmful software into the system. The Teams App Marketplace contains curated apps by Microsoft, any other apps that are not part of the catalog should be considered unsafe.
By selecting the "Block Custom Apps" option, will prevent your user from installing custom apps, which are not part of the list approved by Microsoft.
In the Set section, you can configure one of the following options when you apply the setting:
- Block Custom Apps: Custom apps are blocked in all Teams permission policies
- Allow Custom Apps: Custom apps are allowed in all Teams permission policies
- Do not modify (Ignore): Office Protect will not monitor nor attempt to modify the permission policies on the organization. We recommend using this option if your organization needs to allow a subset of custom applications, or if many permissions policies are set with different Custom apps values.
You can find this setting in the Teams admin center > Teams apps > Permission policies.
Microsoft’s Documentation:
https://learn.microsoft.com/en-us/microsoftteams/settings-policies-reference#permission-policies