How can you put a user profile item in a category on the user's account information page?
In the meantime I've figured it out. The categories are also defined in the 'view' operation by using the type 'user_profile_category'. For example:

$account-&gt;content['my_cat'] = array(
          '#type' =&gt; 'user_profile_category',
          '#title' =&gt; 'Plants Summary',
         );