Subscription vs one-time payment (+ maintenance/updates payments) are two ways off offering to pay.
Freemium just means offering a free tier to let users try the software and offering a way to upgrade to unlock features on paid plans.
B2C vs B2B is business to customer vs business to business.
SaaS is software as a service, as opposed to say installing an application on premise in a data center.
A startup could be offering a fremium B2C SaaS for example, or a freemium B2B SaaS.