Hello Lukáš,
I imagined that component doesn't need to know in which project/product is used, even when in may be intended to more projects/products.
Usually, projects (collections of environment files) are applied at the product level. Sometimes, however, you want to apply one at the component level, so you put a \project in the \startcomponent. You never need to put \product anywhere. Clearer descriptions of what the commands \project and \product do can be found on the command pages; same for all the other structure commands. The [[Project structure]] page is indeed confusing, it needs a better structure itself. Description from http://wiki.contextgarden.net/Command/project: When used inside a \startproduct or \startcomponent environment, \project is an instruction to source any \environment files linked in the project file. Description from http://wiki.contextgarden.net/Command/product: The command \product does not do anything: to link a product to a project file, one should use \project inside the product. Nonetheless, the \product command is often used inside project files as a reminder of what products the project involves. Hope this helps. Kind regards, Sietse