Configuration Manager boundaries will affect the client’s resident (local) management point (MP)
A client’s Assigned MP is the default MP installed on to the site the client is assigned too.
A client’s Resident MP is the MP nearest to the client. (If a client is in the boundary of one site, then the resident MP is the MP installed on that site; A resident MP could also be a proxy MP)
When a client starts up, it will check for its assigned MP and resident MP from the DC. If a client can find resident MP, it will use resident MP. If a client cannot find the resident MP, it will then use the assigned MP.
When the client needs to find DPs for a package, it will first check its resident MP. If resident MP gives it an empty list, it will fall back to its assigned MP for DP locations. If the resident MP gives it a DP list, the client will try the DPs one by one. If none of these DPs works, the client will retry before failing. The client will not fall back to its assigned MP again for DP locations.
The fallback only happens in one SCCM hierarchy. If the client happened to fall in the boundary of the SCCM site of another hierarchy (this could be caused by wrong boundaries setting), the client will not fall back to its assigned MP.
By default, advertisements/Deployment templates are configured with the option "Do not run" when the client is in "slow or unreliable network. Clients that are not in any boundaries are treated as if they are in "slow or unreliable" network. Therefore, by default, these clients would not be running advertisements unless that flag is specifically unchecked.