With a Thatch plan, Thatch is your Broker of Record, handles monthly payments to your carrier, and can help resolve plan issues. With BYOP, Thatch isn't your broker and can't access your policy details.
The most important distinction is that with a BYOP, you pay your carrier directly and are responsible for keeping the coverage active. You’re also responsible for making any changes to your coverage and then informing Thatch of those changes to update your Thatch account to match.