Peter Crosland wrote:
They have the same status as a STOP sign and there does not need
to be any other for them to take effect.

Richard J replied:
In what way does a box junction have the same status as a Stop sign?
You don't have to stop *before* crossing them, and stopping *on* them
is allowed if you are waiting to turn right across oncoming traffic.

"Peter Crosland" then wrote:
In the sense that it is a road sign. Parking restrictions and bus lanes
require statutory notices to be published to allow them to be enforced.

Many road-signs (as well as markings) DO require statutory notices
(Traffic Orders) to make them enforceable, for example a 'no right
turn' sign does. So does a 'no entry' sign. But a yellow box
doesn't.