When filing a trademark application, if the USPTO Examining Attorney issues an initial refusal because he or she finds that there is a substantial likelihood of confusion with a pre-existing mark, this can be an issue with respect to getting the application registered.  One way around this is to enter into a Consent Agreement with the holder of the pre-existing filed trademark.

The holder of the pre-existing filed trademark may not have any incentive to enter into such an agreement, so monetary compensation may be required.  As their are usually parameters that are agreed to, such as use of the marks in certain industries, on certain products, in certain markets, using fonts, colors, etc.

Something to remember is that if you file the Consent Agreement with the USPTO it is a public document and can be viewed by others.  This is an issue if you are paying monetary compensation.  A way to avoid this is to draft two agreements (or one with all of the issues and the compensation in a side letter agreement).

The existence of a Consent Agreement when an initial refusal for confusion is usually found by the Examining Attorney to be a good factor in allowing the filer to have its application registered.  The holder of the pre-existing filed mark is a good barometer for what would be seen as confusion by consumers in the marketplace.