Assigning pools, senders and brandings
On a campaign, Assign pool adds a pool and sets, for that assignment, which sender and branding to use. Leave them on default and the account's defaults apply; for a campaign an agency distributes, client default means each client's own sender and branding are used at send time.
Order matters only among nurture pools (they take turns in this order). Alert and scheduled pools are picked by their own rules; see How the cascade picks the next message.