a.IN AID OF LEGISLATION (sec 21)
President and the SC Justices are NOT compelled to appear.
If summon by Congress under sec 21, cabinet members cannot refuse to appear and can be held in contempt.
The continuance of such incarceration only subsists for the lifetime, or term, of such body. Once the body ceases to exist after its final adjournment, the power to incarcerate ceases to exist as well. Thus, each ‘Congress’ of the House lasts for only 3 years.
In order for a witness to be subject of incarceration, the primary requirement is that the inquiry is within the scopre of Congress powers (i.e. its in AID OF LEGISLATION)