How to combine or coordinate drawing using xref command like mechanical ,electrical,and fire fighting & plumbing?

You can use the xref command then click "attach dwg" or whatever you want.

Dear It's very easy first of all you need to confirmed that your all drawings are linked thru X-REF then you can go on File Manu and Select the eTransmit then add more files and go on modify and choose the Bind option and give location for file then come back and just say ok it will be combine all drawings.

For the X-Ref you will write from keyboard XR then you will select attach file option then go on particular link where is your drawings are located then select the drawing after that it will be ask to you to Relative Path or Full Path just give the Relative Path then say ok then it will be ask you about the Coordinates0,0 point just put from keyboard0,0 then press the Enter key from keyboard it will be done inshah allah

But make sure your drawings Base points are same.