This function returns the GLN of the trading partner. When you call the function without parameters, the trading partner of the current document is used. If you pass a GLN, the location will be searched by the GLN and then the GLN of the partner will be returned.
GetTradingPartnerGLN(['LocationGLN'])
GetTradingPartnerGLN()
GetTradingPartnerGLN('4043077000005')