On campaign Settings → Senders, Rotation strategy is Round robin, Least used today, or Sticky (reuse prior sender). The control is always there. It matters once more than one sender is selected.
Pick a strategy, then save
Default is Round robin.
Open the campaign and select the Settings tab.
Scroll to Senders.
Select the senders this campaign may use.
Set Rotation strategy to Round robin, Least used today, or Sticky (reuse prior sender).
Save the senders section.
If the campaign is active, the page can lock senders until you select Edit campaign. The lock says "Edit campaign to change senders, schedule, or exclusions. You can still enroll leads below."
When two or more senders are selected, the page says "Selecting multiple senders increases daily sending volume so you can reach more prospects per day."
What each option does
Round robin walks the selected senders in turn. Least used today prefers the selected sender with the fewest sends today. Sticky (reuse prior sender) keeps the sender already used for that lead when it can.
A sender still needs a licensed seat and an Active status. No seat is a badge. Account Protection still blocks a send when a reason is true. InMail cannot launch. Use Message.