You can determine separately which inquiries, quotations or orders can be created or changed. Usually, these types are the same but it may be desired that certain types can only be created, changed or displayed.
Order types for order creation
This specifies which new order types can be created by the user.

Order types for changing order
Only orders of this type can be changed by the user.

(This can also be configured for offers and inquiries.)

Order type for availability check
Transaction VA01 is used to check availability. The parameter controls which order type is used here.

Order types for order to offer
An order can be created directly from an offer. Here you define which order types are available.