Only contacts who have been sent an email (not including internal emails) in your workflow and later go on to meet the goal criteria will be counted toward the conversion rate on the Performance tab.
Please note: if you're a Sales Professional user and you don't have a Marketing Professional or Enterprise subscription, while you can use goal criteria to suppress contacts, your contacts won't count towards the goal conversion rate because you need to send marketing emails in workflows to get contacts counted towards this rate. Read more about using goal criteria for sales here.
The goal conversion rate and number includes contacts who complete the workflow after having been sent an email in the workflow and later meet the goal criteria, even though they are no longer in the workflow.
If a contact meets the goal criteria while still active in the workflow, but they haven't been sent an email from the workflow, they will still be unenrolled from the workflow but they won't count toward this number. They also won't meet any list criteria for Workflow status | Contact has met the workflow goal | [Select your workflow]. An email must be sent to the contact; this does not include internal emails sent via internal email actions in the workflow.
Contacts who meet the goal criteria while active in the workflow will still be counted towards the "met goal" number even if they later no longer meet the criteria.
If a contact is unenrolled by some means other than by meeting the goal criteria, they will not count towards the goal conversion rate. For example, if a contact is unenrolled from the workflow as a result of joining a suppression list, is unenrolled if they no longer meet the workflow's starting condition, or is unenrolled after being enrolled in another workflow, and they later meet the goal criteria, they will not be counted towards the goal conversion number.