What cross-subscription communication & connectivity is possible in Azure?

I work in a large distributed org, and several divisions have purchased their own subscription to Azure.

I am a co-administrator to all the accounts, and can see all the resources in all subscriptions.

Is there a per-resource description of what can (and can't) be shared among subscriptions?

For example, the DBA team purchased their own tenant, as did the networking team. My goal is to

• Configure the application gateway so that it routes correctly among all the resources
• Connect websites on premise, and other websites in a different subscription to the DBA tenant
• etc... (Redis)

It seems that some resources are tightly coupled to the tenant, however others less so.

Is there any resource, guideline, or rule of thumb that I can use to understand what can, and what can't be shared amongst Azure Subscriptions/ Tenants?