Selecting a subdomain to filter the Sources report will rebucket visits, contacts, and customers based on how people found that particular subdomain only.
If you see that significantly fewer contacts are being displayed when filtering by All Domains vs. a particular subdomain, try including offline sources when viewing All Domains by checking the Show offline sources box. It's likely that there are contacts whose original source for all domains is offline, while they may have reached the particular subdomain you're filtering by from a different source.
- You imported a list of contacts who were email subscribers on your old email marketing software. Mary Jackson is one of those imported contacts, so she will appear in Sources under All Domains + Offline Sources.
- Mary is also one of your Twitter followers and she sees your most recent blog post in her Twitter feed. Your HubSpot blog is hosted at blog.your-company.com. Mary clicks the link in her Twitter feed taking her to the full blog post in her browser at blog.your-company.com/interesting-blog-post. Liking what she is reading, Mary fills out the blog subscription form on the blog page to become a subscriber of your blog.
- Now that Mary has converted on the blog subscription form after having visited from Twitter, Mary will still appear in Sources with the offline sources box checked under All Domains, but she will also now appear when you filter via the subdomain blog.your-company.com in the Social Media category.