What is the use of Dynamic Attribute in a Rest Channel Adapter in PO?

Hi Masters, im working with Rest Services in PO, but i dont understand what is the use of dynamic attribute in the tab Rest Resources, as you can see in the next image:

I found that configuration on a channel, but I do not know what it does.

Can you please help me to understand what is the use of "id_part", and if I want to test my interface using a dynamic attribute, how would I do it?, to test i use postman or soapui, but i dont know how test when i have a dynamic attribute.

Thanks