The external variables are different from global variables, here's a recap :
The global variables are variables used in the chatbot such as a support phone number, a postal address, a product price and so on. They are common to every user (your contact phone number is the same for everyone for instance).
The external variables are users variables specified by yourself when you integrate the chatbot to your website. It allows you to transmit the chatbot's datas you own to the user. For instance, the user is a client and he connects to your website. You may then send him his client number so as it's used in the chatbot scenario.
Going further :