It comes from EU law. E.g. Council Regulation No 269/2014 defines economic resources as "assets of every kind, whether tangible or intangible, movable or immovable, which are not funds but may be used to obtain funds, goods or services." (https://finance.ec.europa.eu/system/files/2020-06/200619-opi...)

And of course "EU-sanctioned entities" is a rather well-defined list, details here: https://data.europa.eu/data/datasets/consolidated-list-of-pe...