Single instance
A single instance form is a form that only allows a single copy to be open at a time. Single instance forms can be either modal, meaning that while the form is open, no other form can be opened or used, or non-modal, meaning that while only one copy of the form can be opened at a time, you are still free to use other forms while that form is open.
An example of a single instance modal form is the Bank account details form, which once opened stops you from using the Accounts form until the form is closed.
An example of a single instance non modal form is the Manufacturing management console form, which will only allow one copy of itself to be open at any given time, but will not stop other forms from being opened while it is being used.