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.
Brand Kit
The campaign has a Brand Kit (top right of the pools toolbar); a pool row can override it in its Brand column. Left on Default, the account's default kit applies (for an agency, each client's).
The same pool for another audience
Add lists pools already on the campaign under Add again for another audience. The second row gets its own filter, sender, Brand Kit and Blocks; each contact still gets each message once. When only the blocks differ, use variants on one row instead. A row that can never send is flagged at the top of the campaign: an earlier row of the same pool has no filter, or the same filter, so it admits those contacts first. See Blocks: one pool, different content per audience.
Blocks
The Blocks column sets which Block fills each block section of the pool's templates on that row, and how its entries go out. It reads None (each message uses its own content), Single (one section filled from a Block), Multiple (several, no variants) or Custom (variants change the blocks by audience). Hover it for which sections are set. Clicking it opens the row's versions as a table: Default first, then each variant in the order they are checked. Click a version to edit it: pick the template, set its blocks, and for a variant set who gets it in the filter beside them. See Blocks: one pool, different content per audience.