The accounts and folders to be checked are configured in Account preferences.
How manual mail check works
Pressing the button checks mail in INBOX folder of all accounts, where manual mail check is enabled in Account preferences.
Updating other folders
It is possible to check mail also in other folders than INBOX in IMAP accounts.
However, these are updated in limited frequency, not necessarily each time when Update button is pressed.
Choosing IMAP folders for manual updating
To configure which folders are to be manually updated, and maximal updating time frequency, use again Account preferences, and in expandable Manual check mail option you can find another option to allow updating other folders, check frequency, and selectable collection of special folders which shall to be updated.
How are then other IMAP folders manually updated
ProfiMail knows last time when a folder was synchronized with server (and this is also displayed below folder). Each time you press Update button, ProfiMail updates mail in INBOX of account, and in each special folder for which updating is enabled, if the time since its last update is greater than what is configured as Check frequency.
This results in most important folder INBOX to be checked each time when you choose to manually check mail, and other, less important folders, to be updated occasionally.