Your forklift prototype has to be changed. Therefore, you defined all related SAP objects inside one change number. Due to technical reasons, the material BOM has to be changed one week earlier. You would not define a new change number for this particular object. Therefore, you define an alternative date for the material BOM.
Your performed the object changes. Due to technical reasons, all changes have to take place one week earlier. This is why you do a date shift inside the change number.
When you create a change number, you first enter the valid-from date. This date information is then valid for all objects of the change number.
However, you now have the situation that the individual objects want to provide an individual "valid-from" information.
Watch the following video, which explains the use of alternative dates.