Session cookie
A session cookie has no expiry date and is deleted when the browser session ends, so it only lasts for a single visit.A session cookie is set without an Expires or Max-Age attribute. The browser keeps it in memory and discards it when the session ends, usually when the browser closes (some browsers that restore tabs keep them longer). Websites use session cookies to tie together the requests of one visit: the login state, the shopping cart, a CSRF token, load-balancer routing or the steps of a form.
Most session cookies fall under the strictly necessary exemption of the ePrivacy Directive, because the visitor asked for the service they support. The Article 29 Working Party's Opinion 04/2012 lists user-input, authentication, security, multimedia player and load-balancing session cookies as exempt from consent. They still belong in the cookie policy, so visitors know they exist and why.
The lifetime is not what decides the legal treatment. A session cookie whose purpose is to serve advertising or to track the visitor for a third party is not strictly necessary and needs consent, however short it lives; a persistent cookie that stores the consent choice is exempt. Consent tools therefore sort cookies by purpose and only use the lifetime as information for the visitor.