When sending an email, you can select one or more groups that you don’t want to send it to.  If there is any overlap between the distribution list and contacts not on that list, not being in a distribution list will take precedence. In addition, fixed system groups (unsubscribers/hard bounce/soft bounce/spam complaints and people on the exclude list) will not receive any emails.
For example: the distribution list contains 1,000 contacts. The ‘non-distribution list’ includes 500 contacts. If, in the first file, 300 contacts match the ones in the second (don’t send to) file, the email will be sent to 700 contacts on the distribution list and the other 300 that overlap with the ‘non-distribution list’ will be excluded.
If there are also 100 contacts from the system groups among the 1,000 contacts, and if there isn’t any overlap between the distribution and ‘non-distribution’ lists, the email will be sent to 900 contacts from the distribution list, and the 100 contacts from the system groups will be excluded.
The same principle applies when using filters and languages: excluding contacts always takes precedence.