What dhx_globalImgPath means?

I’ve seen

window.dhx_globalImgPath = '/codebase/imgs';

what does it actually mean ? Does it mean that it set every dhtmlx component image path to /codebase/imgs ?

Do I have to do setImgPath for the same path after I used the above (e.g. grid.setImgPath or calendar.setImgPath ?)

I would be better to use dhtmlx.image_path instead. However, dhx_globalImgPath can be used for some components too.
dhtmlx.image_path sets image path for the components. For example path to the image of Combo arrow and sorting icons of Grid. However, Calendar 3.0 do not use img tags, it uses divs with background-images which are defined in the css libraries. Other components are mostly styled by background images as well as Calendar.

At the moment, i use

dhx_globalImgPath 

in a layout file which is rendered on everypage.

I notice that the above does not work with calendar as you said. I have to do setImgPath on dhtmlx calendar object.

So can i do

dhtmlx.image_path

and do not need to worry about setImgPath anymore ?

Yes, dhtmlx.image_path is applied to all components.