@Ca***
Repatriation flights are booked by the embassy. The embassy contracts with an airline to fly a specific route that is otherwise unavailable. You have to book and pay for your ticket through the embassy.
The "semi commercial flights" can be booked directly through the airline or travel/booking agent. They fly normally and are not contracted with any embassy or government. The reason they are called semi commercial and not commerical flights is because the Thai government has suspended commercial air travel into Thailand pursuant to their emergency decree and needed something to call the commercial flights that are flying into the country.