Drupal is built with permission sets in mind.
User may have 1 or more roles.
Content may be limited based on content type, or up to single node level (1record).
When a user is viewing a node / page,
user will have to pass through access "access content".
Next, node may be limited based on content type in:
hook_access
The type of access names can be declared in:
hook_perm
To overwrite permission, you may edit hook_access within the same node module,
or use hook_nodeapi
stages:
* check for "access content"
* trigger nodeapi.op = "load"
* check for hook_access
* running "hook_view"
* trigger nodeapi.op = "view"
* ...
Please help to add or fix this content if you find any mistake or lack of information in this post.