A subform is a form within a form. The primary form is called the main form, and the form within the form is called the subform. A form/subform combination is often referred to as a hierarchical form, a master/detail form, or a parent/child form.
Subforms are especially effective when you want to show data from tables or queries with a one-to-many relationship.
A subform can be displayed as a datasheet, as in the preceding illustration, or it can be displayed as a single or continuous form. A main form can be displayed only as a single form.
A main form can have any number of subforms if you place each subform on the main form. You can also nest up to ten levels of subforms. This means you can have a subform within a main form, and you can have another subform within that subform, and so on. For example, you could have a main form that displays customers, a subform that displays orders, and another subform that displays order details.