There is a (A) regular prisoner, that the normal legal rules apply to, including for terrorists.
Then there is (B) prisoner-of-war, which is for if there is a state of war and said person is a soldier in an enemy state.
Anybody who does not fit into category (B) by default fits into category (A).
"Enemy combatant" is something the Bush Administration pulled out of their ass because they wanted to torture prisoners.
If they were following the rules instead of making up criminal shit like that, they'd have had to treat the AlQaeda suspects ("suspects" including innocent civilians turned in for bounties) as category (A) civilian prisoners and not torture them. That wouldn't have fit the Bushies' plan.
The plan was to torture the prisoners by water boarding and make them "confess" that Saddam Hussein was behind 911 - to manufacture a "reason" for the Bushies to attack Iraq. And the plan worked.