fieldstr vs fieldnum whats the difference?

Is there any difference between fieldstr and fieldnum?

QueryBuildDataSource qbds;
qbds = this.query().dataSourceTable(tableNum(Test));
qbds.name('Test');
qbds.addRange(fieldNum(Test, CustGroupId)).value(strFmt('((%1.%2 == "20") || (%1.%3 == "30"))', qbds.name(),
fieldStr(Test, CustGroupId),
fieldStr(Test, CustGroupId)));

Code doesnt work if i replace fieldnum with fieldstr.