This is what MNPS Operations Manual has to say about SLOP:
Strategic Lateral Offset Procedures (SLOP)
8.5.1 ATC clearances are designed to ensure that separation standards are continually maintained for all traffic. However, the chain of clearance definition, delivery and execution involves a series of technical system processes and human actions. Errors are very rare but they do occur. Neither pilots nor controllers are infallible. Gross Navigation Errors (usually involving whole latitude degree mistakes in route waypoints) are made, and aircraft are sometimes flown at flight levels other than those expected by the controller. When such errors are made, ironically, the extreme accuracies of modern navigation and height keeping systems themselves increase the risk of an actual collision. Within an SSR environment the controller is alerted to such errors and can, using VHF voice communications, intervene in a timely fashion. This is not the case in Oceanic airspace, such as the North Atlantic, where the controller’s awareness of traffic disposition is reliant largely upon pilot voice position reports and communications utilise HF or SATCOM Voice through a third party radio operator. Consequently, it has been determined that allowing aircraft conducting oceanic flight to fly self-selected lateral offsets will provide an additional safety margin and mitigate the risk of traffic conflict when non-normal events such as aircraft navigation errors, height deviation errors and turbulence induced altitude-keeping errors do occur. Collision risk is significantly reduced by application of these offsets. These procedures are known as “Strategic Lateral Offset Procedures (SLOP)”.
8.5.2 This procedure provides for offsets within the following guidelines:
a) Along a route or track there will be three positions that an aircraft may fly: centreline or one or two miles right
b) offsets will not exceed 2 NM right of centreline
c) and offsets left of centreline must not be made
8.5.3 Distributing aircraft laterally and equally across the three available positions adds an additional safety margin and reduces collision risk. This is now a standard operating procedure for the entire NAT Region and pilots are required to adopt this procedure as is appropriate. In this connection, it should be noted thaDistributing aircraft laterally and equally across the three available positions adds an
a) Aircraft without automatic offset programming capability must fly the centreline.
b) Operators capable of programming automatic offsets may fly the centreline or offset one or two nautical miles right of centreline to obtain lateral spacing from nearby aircraft. An aircraft overtaking another aircraft should offset within the confines of this procedure, if capable, so as to create the least amount of wake turbulence for the aircraft being overtaken.
c) Pilots should use whatever means are available (e.g. TCAS, communications, visual acquisition, GPWS) to determine the best flight path to fly.
d) For wake turbulence purposes, pilots should also fly one of the three positions shown above. Pilots should not offset to the left of centreline nor offset more than 2 NM right of centreline. Pilots may contact other aircraft on the air-to-air channel, 123.45 MHz, as necessary; to co- ordinate the best wake turbulence mutual offset option. (Note. It is recognised that the pilot will use his/her judgement to determine the action most appropriate to any given situation and that the pilot has the final authority and responsibility for the safe operations of the aeroplane. See also Chapter 11, paragraph 11.5.) As indicated below, contact with ATC is not required.
e) Pilots may apply an offset outbound at the oceanic entry point and must return to centreline prior to the oceanic exit point.
f) Aircraft transiting radar-controlled airspace mid-ocean should remain on their already established offset positions.
g) There is no ATC clearance required for this procedure and it is not necessary that ATC be advised.
h) Voice Position reports should be based on the waypoints of the current ATC clearance and not the offset positions.