If you remove a product from a subgroup, that action will not remove the product from the product group that contains the subgroup.
For example, if you remove a product from a subgroup named "White
chocolate", that product will still be a member of the "Chocolate"
product group that the "White
chocolate" is a subgroup of.
If you remove a product from a product group, that action will remove the
product from any subgroups under that product group that the product
might be a member of. For example,
if you had a product in a subgroup named "White chocolate", that
product is automatically a member of the product group named
"Chocolate". If you were to remove the product from the
"Chocolate" product group, that
product will automatically be removed from the
"White
chocolate" subgroup also.