A person who works part of a career abroad can end up paying into two national systems and qualifying under neither. Bilateral agreements exist to address both halves of that problem.
The double contribution problem
Most countries levy social insurance contributions on work performed in their territory. Some also levy them on their own nationals working abroad.
An employee posted overseas can therefore owe contributions in both the home and host systems on the same earnings, with no relief through ordinary tax treaties.
Income tax treaties generally do not cover social insurance, which is why a separate class of agreement developed for this purpose.
How coverage is assigned
These agreements set rules deciding which country's system covers a given worker, so contributions are owed in one place rather than two.
The general rule assigns coverage to the country where the work is performed. A detached-worker exception keeps a temporary posting under the home system for a limited period.
The exception has a duration limit, and exceeding it moves coverage to the host country, which is why posting lengths are often set with the limit in mind.
The qualification problem
Benefit systems typically require a minimum period of covered work before any entitlement arises. A career split across countries can fall short in each.
Agreements address this by allowing periods in one country to be counted toward the qualifying requirement in the other, so a split record still opens entitlement.
Counting periods for qualification is not the same as counting them for amount. Each country generally pays only for the contributions actually made to it.
How the benefit is calculated
Where credits are combined to qualify, the paying country typically computes a theoretical benefit and then pays a proportion reflecting its share of the total record.
The result is two smaller pensions from two systems rather than one full pension. The combined amount depends on each country's formula.
Formulas that weight early or low earnings differently can also interact with a partial record, which is one reason outcomes are hard to estimate informally.
What proof of coverage involves
Employers usually request a certificate from the home system confirming the worker remains covered there, which the host country accepts as evidence contributions are not owed locally.
Without the certificate, the host country's authority has no basis to exempt the earnings, and contributions may be assessed while the position is sorted out.
Agreements exist with a limited set of countries, terms differ between them, and the details change over time, so the applicable text is the one that governs any particular case.