Apps installation scope
The normal installation sequence for a SharePoint app is quite straightforward.
A tenant administrator adds the app to the App Catalog in use by the sites within a tenant.
A site administrator adds the app to the web site where it will be used.
Since App Catalog is also a web site, the app is always said to be installed with web scope. An app’s installation scope only refers to how the app was installed. The app developer has no control over this scope.
The key difference between a web scope and a tenant scope app installation is that there is only one app web for a tenant installation. Each site gets a separate app web when using web scope. Consider the impact of using web scope installations when thousands of sites are involved. This would cause the configuration of thousands of new SharePoint web sites that may not be needed.
Consider using tenant installation scopes when the following situations apply:
There will be a large number of sites using the app. Web scope installs create additional copies of the app web.
The sites need to share their data. The SharePoint data stored by the app, including SharePoint lists and libraries, is stored in the app web. If this data should be common to all sites in the tenant, use tenant scope.
The app does not store any data in the app web. In this case, there may be no reason to create the extra app webs for a web scope install.
The app does not expose any app parts or ribbon menu custom actions. Note that custom actions that surface in the Edit Control Block (ECB) can be used within tenant installed apps.