Content types are object types in the product database. Content types in template row definitions automatically filter out unwanted object types from the output template. The current version of Template Editor uses content type listings. The list of content types as well as their effect is product-specific.
When you create a new row in the template, you should select a content type for the row. The content type determines which template attributes can be used in that row.
See also
Modifying the content type of a row in Template Editor
Available content types in Tekla Structures (external link)