parentsearchcode is the product's parent identifier, its value often being your SKU that represents multiple product search codes / variants. 

It serves to identify a group of products that are similar to each other, for example, if you have separate SKUs for different colours of the same shirt (product), you may want to display the rating for the shirt (any colour) (i.e. the parent product) instead.

Where products have been uploaded to Feefo with a parentsearchcode, this value can be used as part of an URL to add a parent's rating rather than an individual product's rating to your website or to generate reports that are based around the group of products rather than the products themselves.
Entersaleremotely API: productsearchcodeEntersaleremotely API: gtin