Partner acquisition refers to growing your partner program by enrolling new partners into your partner program. This involves your partner recruitment strategy and the process it takes to apply to your partner program.
Any marketing channel where you "pay to play," including display advertising, content syndication, search advertising, etc.
A partner join source indicates the channel from which a partner applies to your program, whether from your owned channels (e.g. your website, email, paid ads, internal team outreach), referrals from existing partners, or through a partner network.