Hello hamed;

Here is a Siemens document that is even briefer, but to my mind better structured and therefore clearer.

https://support.industry.siemens.com/cs/document/23451499/what-is-the-meaning-of-the-terms-sinking-(german%3A-p-schaltend-)-and-sourcing-(german%3A-m-schaltend-)-in-digital-modules-of-simatic-?dti=0&lc=en-WW

It relates the signal type, the electronics of the digital module and the wiring of the sensor to the terms sink and source.

There are many ressources available on the web discussing the subject: here is one example from Proface (Xycom):

http://www.profaceamerica.com/sites/default/files/cms/cms/resource_library/app_notes/d04d3f022ec61680/apnt109.pdf

Hope this helps,

Daniel Chartier

Edit: My appologies to Online Tech for repeating the same link he proposed.

D.C.