A session is a bounded period of two-way conversation between a customer and an agent, used by some platforms for pricing and by analytics to group messages into one interaction.
Sessions in pricing
Some carriers and providers outside the US price RCS by conversation: one fee opens a window, typically 24 hours, during which further messages in that thread are included. Others, and US pricing generally, charge per message, with a different rate for a basic text and for a message carrying a rich card or media. The two models suit different traffic; a chatbot exchange of thirty short turns is cheap per session and expensive per message, a single reminder is the reverse. The cost guide sets both models side by side, and the SimplyRCS rate card is per message with no session fee.
Sessions in analytics
Independent of billing, platforms group messages into sessions to answer questions like "how many turns did it take to resolve" or "what share of conversations ended in a booking". A session usually starts with a mobile originated message or an agent message and closes after a period of inactivity.
What a session is not
It is not a consent window. A customer's consent governs whether a business may message at all and does not expire with a session; a STOP ends the relationship, not the conversation.