Transaction processing is not refering to a group of operations and also is not a operation itself. It is kind of creating the transaction itself.
Let me explain it with an example:
- Account A (thresholds low=1, medium=2, high=2, two signers with weight=1 each)
- Account B (default)
- Account C (default)
Account A creates a payment transaction on behalf of B who sends some cash to C (the transaction is sourced from A, it contains one payment operation from B). Although A is a multisig account it needs only one signature because the act of crafting a transaction itself is low weight. Account A will be charged the XLM network fee and A will get its sequence number increased by one. (Obviously B will also need to sign the transaction cause it's his money)
The only low weight operations are
BumpSequence as stated in the docs.