Failed to apply background mesh command

[qipanymrs]

Dear Dr. Zhu,
Thanks for your latest updates of Openseespy Documentation. I tried to follow the example in your PEER report with background mesh method. But it failed. I have some doubts with this command application.
Firstly, is the background region the fluid domain or the computational region?
secondly, is lower (wlower) and upper (wupper) the coordinates of node we defined or the nodes generated from mesh command? How to define these parameters?
Could you give me any suggestions or some example scripts?

Thank you

[zhuminjie]

background mesh commands will be updated shortly. Thank you!