README.txt in Service Container 7.2

Drupal Components are independent libraries that do not depend on the rest of
Drupal in order to function.

Components MAY depend on other Drupal Components or external libraries/packages,
but MUST NOT depend on any other Drupal code.

In other words, only dependencies that can be specified in a composer.json file
of the Component are acceptable dependencies.  Every Drupal Component presents a
valid dependency, because it is assumed to contain a composer.json file (even
if it may not exist yet).

Each Component should be in its own namespace, and should be as self-contained
as possible.  It should be possible to split a Component off to its own
repository and use as a stand-alone library, independently of Drupal.