Switch loading is the practice of reloading a tanker compartment which previously carried one of the products petrol (UN1203), kerosene (UN1223) or diesel (UN1202) with another of those products. It is commonest in the fleets that supply retail fuel outlets and so will usually include petrol.
ADR allows the tanker to be placarded/ marked as if carrying only the most hazardous of the substances (descending order of hazard 1203, 1223, 1202)
ADR does not deal well with this practice, but paragraphs 220.127.116.11.5 & 18.104.22.168.1 are relevant. For example, if a compartment that previously contained petrol is left nominally empty, then the placarding and marking for petrol (1203 / 3YE) should remain in place.
Changes to practice for the purpose of controlling emissions of volatile organic compounds means that it is possible for a compartment discharging (say) diesel at a retail filling station to be backfilled with petrol vapour.
Therefore, for all practical purposes, tankers engaged in switch loading will retain their "1203/3YE" panel at all times, and enforcement officers should accept this practice.
This does not mean that road tankers dedicated to carrying diesel or kerosene should be placarded and marked for petrol. Typically these vehicles are dedicated to industrial /agricultural /domestic fuel oils and will often be "rigids". Another category serves the aviation market and these may be marked UN 1223 or UN 1863.
Where, in this context, it is clear that a vehicle is not correctly placarded and marked, enforcement officers should note the guidance on action to be taken (advice only).
When vehicles are taken to workshops or inspection stations, it would be expected that systems of work for any testing or maintenance and repair work involving the cargo tank or its ancillary equipment would not rely on tanker placarding and marking as a basis for decision making.