You can configure the sub grids in the Edit/Detail Forms of any module. You need to drag & drop the sub grid layout to the design layout area (body) from the Widget section. 
You can drag & drop one or more Sub-grid layouts to display the details of the module form. 
By clicking on the Edit icon of the Sub-grid, you can find the Sub Grid Configuration to setup the Sub-grid for the selected module form. You can configure the following settings:
Label: Insert the text label of the Sub-grid section.
Select Relationship: You need to select the Relationship with module. You will get the drop-down list consisting of all the entities relationship. i.e. 1:N and N:N with which the specific module form is related. You can select a relationship module that you want to relate the module form with it.
Select List: You will get the drop-down for the list view that you want to display for that selected relationship type sub-grid.
You can provide the access of the Sub-grid to the portal users. You can manage the access from the Access Setting.
You can enable the access for the Add (Create), Edit, and Delete rights to the portal users to provide the access of the Sub-grid. 
Allow Creating Records: You can enable this option if you want to give the “Create Records” rights to the portal users.
– Button Label: Insert the text label for the create button, the default label will be “Create”.
– Select Form: You need to select the ‘Form’ that you want to provide to the portal users when they create a new record in sub grid.
– Allow Editing Records: You can enable this option if you allow the customers to “Edit” the existing record.
– Button Label: Insert the text label for the ‘Edit’ button, the default label will be “Edit”.
– Select Form: You need to select the ‘Form’ that you want to provide to the portal users when they edit any existing record in sub grid.
– Allow Deleting Records: You can enable this option if you allow the customers to “Delete” any record.
– Button Label: Insert the text label for the ‘Delete’ button, the default label will be “Delete”.
– Submit Button Label: Insert the text label of the “Submit” button. Once the portal users created or edited the Form records, they need to save/update the changes by clicking on this Submit button.
Enable Global Access: By enabling this feature, you can allow portal users to access the global data, by unchecking it, portal users can only access their records in the sub-grid. 
Note: Sub-grid option will apply only for the Edit & Detail Form Type, it will not be applicable for “Use Existing forms” that created in the Portal.
If you have enabled and configured the reCAPTCHA (V2 or V3), you will get the Enable reCAPTCHA option under the Sub Grid Form of any form builder.
Note: Sub Grid Form will support both versions of reCAPTCHA V2 & V3. So, anyone is enabled, the Enable reCAPTCHA option will enable in the Sub Grid Configuration.