Difference between Issue & consumption in Item groups

Hi folks,

In Item group form Invoice group .

We have the Issue , Consumption.

what is the Difference between Issue & consumption in Item groups.

Help says :

Packing slip: Transactions are generated in this account when the item is packing slip updated (issue).

Packing slip offset: Transactions are generated in this account when the item is packing slip updated (consumption).

Could any expert throw with practical example about this.