DMs are direct inbox messages and great for personal, 1-on-1 outreach.
Open Campaign invites are sent through TikTok's affiliate system and allow creators to accept or ignore your standard commission offer.
Target Invites are sent through TikTok’s affiliate system and allow creators to accept or ignore the unique commission offer you choose.
Use both in your strategy: DMs are meant to warm up creators, notify them of upcoming releases, or invite them to groups where you can train/manage your campaigns, while Open/Target Collab Invites are meant to receive sample requests from a variety of creators you filter.