A user who creates a form is automatically assigned to "Collect" and "View" data for this form. If the form will only be used as a sub-form, then you want to remove the "Collect" rights for all users. This will ensure they do not see the sub-form appear in the form menu on the device. They can still collect data for this form, but it must be entered by first opening the parent form.
The subform widget can be used to create sub-section of a larger form. It is more efficient to create smaller "subforms" and place them inside a parent form. Subform widgets can be configured to allow for only a single or multiple paging.
Single Paging: When set to single paging, the user may only capture a single record for that particular subform widget (1-1 relationship).
Multiple Paging: When set to multiple paging, the user may capture as many records as they need for that particular subform widget (1-many relationship).
To set how many sub-form records can be collected for the parent, set "Link Mode" to either "Single" or "Multiple" in the "Smart Paging" options.
For a subform link to be shown in list view of the parent form, the subform must contain data. If the subform does contain any data for that parent record, no link will appear.