A TAC (Total Allowable Catches) is the biggest allowed catch volume for one species of fish in one or more given sea areas. When a TAC is established, it can be distributed as quotas to the associated fishing nations. A quota is a nation's maximum allowed catch volume of one species in a bounded sea area. In some cases, a quota can have more species in it, e.g. Dab and Flounder in the North Sea, as can the sea area be defined differently. One stock of fish will always refer to one or more certain species of fish within a bounded sea area.
There is a certain flexibility in the quota system. As such, a violation of allowed landings in quoted species can be tolerated by up to 5 %. However, such violations effect next years quota on the mentioned specie negatively. It is possible to request the Commision for a permit of landing excess amounts of a stock under a preventive TAC, though at most, 10%.
The excess amount will cause a reduction in next years quota.
The Commision can, for stocks restricted to analytical TAC's, authorise to withhold at the most 10% of the quota, and transfer this amount to the following year. It is also possible for each of the countries in the EU to mutually exchange quotas.
An EU-amount is a quota which is an undistributed and available amount to selected membership-countries. It is possible to have both a national quota and an undistributed quota. In later years most quotas have been distributed, meaning that only a few undistributed quotas are available to member countries of the EU.
Read more about TACs and quotas and find the underlying regulations here:
Europa Commission home page on total allowable catches (TACs) and quotas