From the board you want records to appear as sub-records, create a link to record to any field on a parent board. That would act as the primary field from which you create the sub-record field.

Here, "Goods ordered" is a link to record field

With your primary field ready, Click on the '+' sign to the right of the last field the header on the current board.

This pops up a modal or slider to display all field types (including system-generated ones) you can create on your board.

Here you can scroll through or type Sub-record, select the field type, and input field name and description.

Then select the primary field in which the sub-record board has a unique link to record to

Select the board to act as a sub-record. The board must have a unique link to the record field linking to the primary field on the current board.

Then select the form you want to display the sub-record (either as a number of records or a function value).

To display as function value you need to have a number or money field type as part of your sub-record. You can choose to display functions like the Sum, Average, Median, Minimum, Maximum, Variance, or Standard Deviation.

Once you have all set up, click on “Create Field” to add a sub-record field to your parent board.