It is generally accepted that private international law is a branch of national (internal) law, which governs conflicts between rules of substantive law that should be used for the evaluation of social relations shaped by such laws and procedures. In the case of labour law, the subject of private international labour law are standards, which allow to select and apply labour law standards of particular Member States to assess the legal positions of parties within legal relations regulated by employment law (workers, employers) and social security law (the insured, policyholders and insurers). Labour and social security law is a branch capable of a certain reach only. Within the vast majority of countries, labour and social rights are governed by one system of labour law and one system of social security law. Occasionally, the federal states in their various organisational units, which make up the federation (states, provinces, regions, republics), have within their structure two separate systems of law both for labour and for social security. Establishing a work relationship between parties, of which at least one of the parties comes under the regulations of another member state, either on the grounds of nationality (lex patriae), or residence – domicile (employee, insured) (lex domicilii), or because of the location of the headquarters – situs (employer, insurer, the insurer – the insurance) (lex rei sitae), makes regulation necessary of the content of legal relations governed by different national systems of labour and social security law.