Hi @rjbu
Session ID is a unique ID for a user session. By default, this ID lasts 30 minutes.
You can able to see this unique ID in cookie name called mbox dropping by Adobe target JS library in browser.
In terms of Server side implementation where you're using Delivery API for deliver the experience through different channels, API user needs to generate and maintained the sessionID for the current session The sessionId can be any printable string except a space, ?, or /. It should be between 1 and 128 characters in length.
Hope this helps.