It is a private contractual requirement, stricter than the law, and it means that
detected pesticide residues must not exceed a stated fraction of the legal maximum
residue level for that active substance in that commodity.
The legal position is set by Regulation (EC) 396/2005, which fixes an MRL per
substance per commodity across the European Union. A product below the MRL is
compliant. A retailer who specifies fifty per cent of MRL is not restating the law, it
is imposing a commercial margin on top of it, usually to protect against measurement
uncertainty, batch variability and the reputational cost of an exceedance found by
someone else.
Many retailers add a second requirement alongside it: a cap on the number of different
actives detectable in a single sample, regardless of the level of each. That one
catches suppliers who are comfortably below every individual limit but use a broad
programme.
Both requirements are contractual, both vary between retailers, and neither is
negotiable once the listing is signed. They have to be designed into the grower control
programme, because they cannot be met by testing at the end – by the time a finished
product sample is analysed, the residue is already there.
Check also how the requirement interacts with measurement uncertainty. Some
specifications apply the percentage to the reported result and some to the result
adjusted for uncertainty, and the difference decides marginal cases.