"Check Box-Expenses" vs "Group-Expenses"

When I create a group named “Cost of Sales & Services” in P&L/Chart of Accounts-

it shows to mark the checkbox "Expenses"

But if I select “Expenses” from the drop down list from the label “Group”, the checkbox disappears.

Whats the deal here?

When you assigned the account to the Expenses group, it will be reported there. If you leave it as uncategorized, Manager needs to know where to report it, meaning whether to add or subtract its entries, hence the checkbox. Think of the checkbox as triggering a default group, available in the background for simple charts of accounts without other groupings.