And here’s ICAO Doc 4444
Actions to be Taken once Offset from Track
The pilot’s judgement of the situation and the need to ensure the safety of the aircraft will determine the actions outlined to be taken. Factors for the pilot to consider when deviating from the cleared track or ATS route or level without an ATC clearance include, but are not limited to:
a)operation within a parallel track system;b)the potential for User Preferred Routes (UPRs) parallel to the aircraft’s track or route;c)the nature of the contingency (e.g. aircraft system malfunction) and;d)weather factors (e.g. convective weather at lower flight levels).If possible maintain the assigned flight level until established on the 9.3km (5NM) parallel, same direction track or ATS route offset. If unable, initially minimize the rate of descent to the extent that is operationally feasible.
Once established on a parallel, same direction track or route offset by 9.3km (5NM), either:
a)descend below FL290, and establish a 150m (500ft) vertical offset from those flight levels normally used, and proceed as required by the operational situation or if an ATC clearance has been obtained, in accordance with the clearance; or Note: Descent below FL290 is considered particularly applicable to operations where there is a predominant traffic flow (e.g. east-west) or parallel track system where the aircraft’s diversion path will likely cross adjacent tracks or ATS routes. A descent below FL290 can decrease the likelihood of conflict with other aircraft, ACAS RA events and delays in obtaining a revised ATC clearance. b)establish a 150m (500ft) vertical offset (or 300m (1000ft) vertical offset if above FL410) from those flight levels normally used, and proceed as required by the operational situation, or if an ATC clearance has been obtained, proceed in accordance with the clearance. Note: Altimetry System Errors (ASE) may result in less than 150m (500ft) vertical spacing (less than 300m (1000ft) above FL410) when the above contingency procedure is applied