Relation > Category
As you can link your content to products, you can do the same thing with your categories;
Add a new field → in Input type select Relation - Category
Then you can select the category or categories you want to be linked with your content type.
Title : Label of the field
Identifier : Unique ID, used for template redefinition and custom script. By default, the identifier is set by a concatenation of the Custom Type identifier (eg “all_fields”) and Title (eg. “area”).
Input type : current definition of the field type. Note that after saving, you’ll be unable to change it.
Required : define if the field is mandatory or not when user will complete the content form.
Show in Grid : define if the field will be available to be showed in the content listing (under CMS → Your content type name). Take care because all value will be showed.
Note : A note which will be displayed under the field to help the webmaster, or give information about this field.