An internal transaction is any interaction that a contract does with a specific address (or any internal call within a contract). We listen to the interactions you've set up in your Stream. So if your Stream was set up as a "Wallet Transfer" then there are a couple of points to mention:
1. We only provide internal transactions where there is a transfer of value.
2. We only provide the internal transactions that mention the addresses you're watching.
You can monitor internal transactions happening on chain for your Stream by setting the includeInternalTxs parameter to true.