Before-save versus after-save record-triggered flow capabilities
Key Takeaway
A before-save flow (Fast Field Updates) is faster, but it can only update and validate the record that triggered it. For anything a before-save flow can't do, such as creating a related record, use an after-save flow (Actions and Related Records).
A record-triggered flow runs automatically when a record is created, updated, or deleted. There are two types of record-triggered flow. A before-save flow changes or checks the record that triggered it before Salesforce writes the record to the database. An after-save flow runs after the save, when the record already has an ID. Choosing between them is a trade-off: before-save is faster, and after-save can do more.
Record-triggered flows and the two types
A record-triggered flow runs in the background every time a record of the chosen object is created, updated, or deleted. The flow runs whether the change comes from the Salesforce UI, a spreadsheet import, or an API integration. The record whose change starts the flow is the triggering record.
You set the trigger in the Start element. There you also pick the type: "Fast Field Updates" for a before-save flow, or "Actions and Related Records" for an after-save flow.