NPN and PNP are Bipolar junction transistors. They come in two configurations. PNP has a negative output and NPN has a positive output to industrial controls.
The above question is asking how to connect a solid-state device to a weld control or PLC. Weld control inputs may require sinking or sourcing or just simple relay closures. The machine integrator or designer needs to match the control input type against the sensor output type.
For additional sources the internet will discuss sensor sinking and sourcing and could help understand the differences.
Machine design is beyond the scope of this blog.
Reference: RWMA - Resistance Welding Manual 4th Edition