How does the source drop down gets data depending on Source Medium

Summary: We have Source Information in Candidate Job application. I would like to know from where the drop down values are populated.

Content (required): We have Source Information in Candidate Job application. I would like to know from where the drop down values are populated. I am able to see that for Source Medium - Candidate Created Manually, we are getting the Source drop down values from Setup and Maintenance > Recruiting and Candidate Experience Management > Candidate Dimensions Source Names. But for other source mediums like Job board, career site or referral website, how does and from where Source is getting auto populated with drop down values. Can we customize these sources.