Strategic Lateral Offset Procedures (SLOP) is used in the NAT for:

Wake Turbulence Avoidance: It helps pilots avoid wake turbulence generated by preceding aircraft.

Collision Mitigation: It provides lateral spacing to avoid collisions.

• An aircraft may fly offsets right of centreline up to a maximum of 2 NM.
• Offsets left of centreline are not permitted.

Remaining on the centreline is considered a part of SLOP, as pilots are free to select any distance between 0 and 2 NM to the right in increments of 0.1 NM.

NORTH ATLANTIC OPERATIONS AND AIRSPACE MANUAL 
ICAO Doc 007 
Strategic Lateral Offset Procedures (SLOP)

8.5.8 While ATC clearances are designed to ensure that separation standards are continually maintained for all traffic, errors do occur. Neither flight crews nor controllers are infallible. Gross Navigation Errors (usually involving whole or half latitude degree mistakes in route waypoints) are made, and aircraft are sometimes flown at flight levels other than those expected by the controller. Ironically, when such errors are made, the extreme accuracies of modern navigation and height keeping systems themselves increase the risk of a collision. Within an ATS Surveillance environment where VHF communications are available, controllers alerted to such errors will intervene using VHF voice communications. In areas (surveillance or otherwise) where VHF voice communication is not available, controllers rely on voice and data link position reports augmented by ADS‐C and ADS‐B transmissions to monitor conformance. Controllers, when alerted to errors, will intervene using HF, CPDLC, SATVOICE or any other means available. Given the potential delay in intervention, it has been determined that encouraging aircraft operating in the NAT to fly self‐ selected lateral offsets provides an additional safety margin and mitigates 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)”.

8.5.9 This procedure provides for offsets within the following guidelines:

a) an aircraft may fly offsets right of centreline up to a maximum of 2 NM; and 
b) offsets left of centreline are not permitted.