Services
Acts of the relevant Legislature may regulate recruitment and service conditions for Union/State public services and posts - until such a law exists, the President (for Union services) or Governor (for State services) may make rules on this, which remain subject to any later Act.
This gives Parliament and State Legislatures the primary power to set the rules for government employment - recruitment processes, promotions, service conditions - while allowing the President/Governor to fill any gaps with rules until a proper law is passed.
This is the constitutional basis for the various civil service rules and regulations (covering recruitment exams, service conditions, and career progression) that govern government employees across India.