The core sending/campaign features are the same across all tiers — what scales up is contact/team limits and support level. The Starter tier caps at a smaller contact count and email-only support; Pro raises those limits and adds priority support with faster response times; the top tier removes contact caps entirely and adds a few team-focused features like advanced role permissions and audit logging. If you're not sure which fits, happy to look at your actual usage (contact count, team size) and suggest the right one.