Change Controller requirement for compatibility

Author: alabou

docs/NMOS-With-IPMX-USB.md

@@ -162,8 +162,7 @@ Unless constrained by [IS-11][], a Sender MAY produce any USB stream that is com
 
 
 ## Controllers
-
-A Controller SHOULD use a Sender's `urn:x-nmos:cap:transport:usb_class` capability to verify  Receivers' compatibility with the Sender and, if necessary, constrain the Sender to ensure compliance with the Receivers. A Sender indicates its support for being constrained on this capability by enumerating `urn:x-nmos:cap:transport:usb_class` in its [IS-11][] `constraints/supported` endpoint.
+A Controller MAY use IS-11 active constraints and the Sender's `urn:x-nmos:cap:transport:usb_class` capability to constrain the Sender to ensure compliance with the Receivers. A Sender indicates its support for being constrained on this capability by enumerating `urn:x-nmos:cap:transport:usb_class` in its [IS-11][] `constraints/supported` endpoint.
 
 > Note: There is no `usb_class` Sender attribute, as might typically be expected, because a USB stream is composed of multiple sub-streams, each of which can be associated with multiple USB classes. The set of classes present in a given USB stream often changes dynamically.