What determines whether a template is an individual, organsation or combined constituent template is the value of the key indicator. In the template coding, if the key indicator is default to "I" or to "O" then that field is not shown and the template is assumed to be of that type. If the key indicator field is mapped to an incoming field then it is assumed to be combined.
If you select combined (indicating that your file has both individuals and organisations), but you then select a default value of either I or O, the template will revert to the setting that you selected as a default.
If you need help setting i[ combined template, see this knowledgebase article for information about how to use data transformations to determine if a record is an I or an O.