Understanding the difference between serial generation

Understanding the difference between serial generation 'At Receipt' v/s. 'At Sales Order Issue'?

Here is an explanation of the serial generation flag

At inventory receipt:

Create and assign serial numbers when you receive the item. Thereafter, for any material transaction, you must provide a serial number for each unit.

At sales order issue:

Create and assign serial numbers when you issue (ship) the item against a sales order. If you select this option, serial numbers are required at ship confirm. If you receive an item on an RMA (return material authorization), you must specify the same serial numbers you created at sales order issue. All other material transactions for this item bypass serial number information.

No control:

Serial number control not established for this item. All material transactions involving this item bypass serial number information.

Predefined:

Assign predefined serial numbers when you receive the item. Thereafter, for any material transaction, you must provide a serial number for each unit.