Different forms of domestic support have different effects on international trade. To help compare different forms of support and the resultant effect they have on trade, the WTO came up with a system to help categorise domestic support. Payments are now placed into hypothetical "Boxes" and the definition of the various boxes is reviewed in agricultural negotiations. Countries are restricted, under WTO trade rules, to only having a certain percentage of the total value of agricultural production as support in certain boxes (although there are exceptions).
The Green Box - Domestic Support eligible for this box must not distort or at most cause minimal distortions on international trade. These payments must be government-funded and cannot involve price support. There is no limit to the amount of support that can be placed in this box. The definition of "minimal" depends upon the interpretation of members. Negotiation has occurred over whether certain methods of support such as decoupled income payments should be eligible for this box.
The Blue Box - This box contains support that is significantly distorting but also involves production-limiting measures. For instance, the domestic support described by the Common Agricultural Policy of the EU after the MacSharry Reforms of 1992 would have been placed in the Amber box if it had not included legislation for compulsory set aside of productive land. There is currently no limit to the amount of support that can be put into this category.
The Amber Box - All domestic support measures that cause a significant distortion of international trade are placed into the Amber Box (with some negotiated and Blue Box exceptions). Support payments in this box are subject to ceiling levels which are referred to as the "de minimis." For instance, in developed countries, amber box support can make up no more than 5% of the value of agricultural production. Note that any trading countries that exceed amber box support levels may face WTO punishment.
The Red Box - This is the category where domestic support that is completely out-lawed is placed. Direct production subsidies, purely framed and designed to boost domestic production, are an example of such support. No agricultural trading nations use domestic support that could be placed into this box. Instead, the box acts as a "list" of entirely illegal forms of support.
The Development/S&D Box - These are pre-negotiated exceptions to the amber box. They are sometimes justified in terms of the effect support may have on development or other Non-Trade Concerns (NTCs). For instance, some domestic support of developing countries, designed to protect their foetal industries, is placed into this box. | 
