VAT gap distribution in Belgium 2017, by category

The source sates the following about the policy gap, rate gap and the exemption gap:

The policy gap is an indicator of the additional VAT revenue that a MS could theoretically (i.e. in the case of perfect tax compliance) generate if it applied a uniform VAT rate on all goods and services.

The rate gap is defined as the difference between the VTTL and what would be obtained in a counterfactual situation, in which the standard rate, instead of the reduced, parking, and zero rates, is applied to final consumption. Thus, the rate gap captures the loss in revenue that a particular country incurs by adopting multiple VAT rates instead of a single standard rate.

The exemption gap is defined as the difference between the VTTL and what would be obtained in a counterfactual situation, in which the standard rate is applied to exempt products and services, and no restriction of the right to deduct applies.