A product can be set up to be a Group.  If an item is a group, the items included in the group will be individually deducted from inventory when the group product is shipped.

A group product can only have one combination.  If the item is set up as a group you will not be able to establish multiple combinations.  Likewise, if multiple combinations of a product have been established, you will not be able to change a product to be a group item.  You can however include a specific combination item in a group product.