what is welding plate?
Probably a mistranslation of soldering. As far as I can tell, it just means that the headers are not soldered on. 

(see https://www.aliexpress.com/store/product/Nano-V3-0-ATMEGA328P-improved-welding-plate-without-wiring-module-board/630494_32807228948.html for one with "welding plate")