Under EASA regs, you will find slightly different interpretations from different countries.
You can either do a SP type rating course and check (if the helicopter is certified for SP Ops), or you can do a MP TR course and check (if the helicopter is certified as a multi pilot helicopter) The third option is to do a MP type rating course/ check on a helicopter which is certified as single-pilot, but is being flown under an AOC for multi-pilot operations. (So, you will need an Ops manual which describes the operation)
On top of this you can do a SP-MP or an MP-SP conversion course and check. For any MP TR, you will need to be MCC qualified first.
Your annual proficiency check could be MP, SP or both. Some authorities will allow you to do both together (obviously you will have to fly some elements of the check without any assistance from another crew member). Other authorities will tell you you have to do 2 checks to achieve this. Depends on which bit of EASA-land issued your licence.