Being curious I’ve looked this up - it’s Article 6 - aggregation. Isn’t EU social security co-ordination wonderful 
"2. Aggregation (Article 6)
This principle guarantees that previous periods of insurance, work or residence in other countries are taken into account in the calculation of benefits. If, for example, national legislation requires a worker to have been insured or employed for a certain period of time before they are entitled to certain benefits, the aggregation principle means that the competent Member State must take account of periods of insurance and employment completed in another Member State."
From an EU factsheet