The home country is the country in which an internationally mobile employee is ordinarily based and from which they are sent on an assignment. The host country is the country where work is physically carried out during the assignment. Determining which country's rules apply across employment law, tax, and social security is the fundamental challenge of international mobility, and the answer often differs between these three legal domains for the same assignment.
Under the EU Rome I Regulation (593/2008), the governing law of an employment contract is generally the country where the employee habitually works. However, even where the parties choose a different governing law, mandatory rules of the country where the work is performed apply to protect the employee. This means host-country minimum wage, working-time rules, and anti-discrimination protections typically apply regardless of the contract's choice-of-law clause.
For tax, bilateral treaties determine whether the home or host country has the primary right to tax employment income, often split by working days. For social security, EU Regulation 883/2004 assigns liability to a single state, usually the host for assignments exceeding 24 months and the home state for shorter postings covered by an A1 certificate. Employers must map all three dimensions separately rather than assuming one set of rules governs the entire arrangement.