AcyMailing does not impose you the exporting format but gives you the possibility to select the fields you want to export.
If you use special characters, you may want to modify the exporting format. The data in your database is encoded in UTF-8, that's why we export in UTF-8 by default but if you see weird characters in your exported file, please select your own encoding format.
The Excel security parameter checks the presence of some special characters that could be interpreted by Excel when opening the export file. This can change the content of fields with these special characters so you can enable the option only if you will use the file in Excel.
Note that if you have applied filters on the listing these filters will be applied to the exported users too.
If you didn't select users in the listing before accessing the export screen, you will be able to choose the lists of users you want to export.
If you select "Export both", AcyMailing will export both subscribed and unsubscribed users of the selected lists (so any user that has been subscribed at one point).
If you select some users on the users listing page before clicking the export button, AcyMailing will only export them as you can see on the above screenshot.