Exchange Rate - Conversion field

Exchange Rate - Conversion field : F1 Says : Select this check box to include the selected currency in the calculations of the online numeric conversion tool. The tool automatically converts amounts that are entered on transaction lines in the company currency to amounts in the selected currency. What is this tool, where can i find in AX and how this tool useful in business scenerio ?

I got the answer of where the tool is located. But i have one question is when we have the company currency and Secondary currency , apart from these how the conversion tool would help in a business scenerio ? Any help would be highly appriciated .