If a contact is tagged as 'new_lead' and 'south-west' (for example) but you wish to send out an email campaign to only contacts tagged as 'south_west' and exclude contacts tagged as 'new_lead', then your contact in this scenario will be excluded from being sent your email campaign as they are tagged with the tag you wish to exclude.
To find out more about excluding tag groups from receiving an email campaign please click here for more guidance.