Deal's value represents total monetary compensation of the deal.
neetoCRM supports multiple currencies so that you can add your country currency.
Deal values helps in forecasting upcoming revenues and introspecting past results.
There 2 ways though which deal's value can be added or updated
Products: Through products, deal's value can be updated. Whenever you add a product, deal's value get updated with product value. Also deal's currency gets updated with product currency. Once you have added a product on a deal then you cannot update deal's value manually ie through Add/Edit deal form.
If multiple products are added on deal with same currency then deal's value would be summation of all products values.
If multiple products with different currencies are added on deal then latest added product currency and value would be considered for deal currency and value. All previous product values would be updated to 0 and their currency would be updated with latest product currency with respect to deal.