The Global tab holds account-level settings that apply across your listings rather than to a single product. It covers two things: how your store currency is converted to your eBay account currency, and your VAT details.
It sets the exchange rate the app uses to convert your Shopify store currency into your eBay account currency. This matters whenever your Shopify store and your eBay account operate in different currencies, since prices listed from your store need to be expressed correctly in the currency of your eBay marketplace.
Setting this accurately ensures your eBay prices reflect the right amount after conversion.
VAT Details
This is where you state whether VAT applies to your products for a particular country and at what rate. The Business seller dropdown indicates whether you're registered as a business seller, and the VAT Percentage field is where you enter the applicable rate, which can be up to 27%.