You’ll need an awful lot more than this to understand the requisition worksheet[:D], but here you go:
New - A new requirement, previously not fulfilled in anyway, it creates a new order for you.
Change Qty - You need to change the quantity on an order as the input or modifiers have altered and you are ordering too many or not enough
Reschedule - A change has occured regarding the date of the demand and the supply should be moved accordingly.
Reschedule & Change Qty - Both of the above events have happened, the source demand has moved in or out whilst the quantities have also altered.
Cancel - You no longer need this so why are you buying/making it. Cancel it.