stay restriction on Z and S1 visas

i am a canadian engineer and will be going to a PRC for short term assignment (7/8 months). my wife and son will join me on S1. i have been told that the Z &S1 types are single entry. does it mean that during the stay we cannot get out of PRC and if we travel outside PRC, then fresh Z and S1 types have to requested/issued? It sounds very strange. please confirm.