The node access system
All node access modules are queried using hook_node_grants() to assemble a list of "grant IDs" for the user.
This list is compared against the table.
If any row contains the node ID in question (or 0, which stands for "all nodes"), one of the grant IDs returned, and a value of TRUE for the operation in question, then access is granted.
Note that this table is a list of grants; any matching row is sufficient to grant access to the node.
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