I want that all read messages are expired (at least in some groups). How to do it?
If you want all read messages to be expired (e.g., in mailing lists where there's an online archive), you've got two choices: auto-expire and total-expire. Auto-expire means, that every article which has no marks set and is selected for reading is marked as expirable, Gnus hits ‘E’ for you every time you read a message. Total-expire follows a slightly different approach, here all article where the read mark is set are expirable.
To activate auto-expire, include auto-expire in the Group parameters for the group. (Hit ‘G c’ in summary buffer with point over the group to change group parameters). For total-expire add total-expire to the group-parameters.
Which method you choose is merely a matter of taste: Auto-expire is faster, but it doesn't play together with Adaptive Scoring, so if you want to use this feature, you should use total-expire.
If you want a message to be excluded from expiration in a group where total or auto expire is active, set either tick (hit ‘u’) or dormant mark (hit ‘u’), when you use auto-expire, you can also set the read mark (hit ‘d’).