Asymmetries are the differences between the trade statistics of partner countries. As each country collects data on their own international trade-in-goods for both imports and exports, the 'mirror flow' (country A's exports to country B, and country B's imports from country A) should in theory match. In practice, the figures do not match and an 'asymmetry' for the period in question can be measured.
Here is a detailed overview of asymm​etries​.
Annual reports explaining the reconciliation of asymmetries in trade-in-goods statistics as published by the UK and other European Union Member States are available below.

Quality assurance archive​