Hi,
Extends means that the package "is" an instance of the package it extends as well as itself. It can inherit templates from the package it extends.
Embeds works slightly differently. In the package you are embedding, the scripts/templates are under package-key/embed. The embedding package then has those scripts avaiable under www/other-package-key.
Implements-subsite-p is specifically for packages that extend acs-subsite (or extend a package that extends acs-subsite, etc). This gives openacs extra information about what packages can perform an acs-subsite role. Mainly this sets the ad_conn subsite_id variable and is used in the site_node procs to find the closest subsite.
