Section 3.7.12 of the JCR 2.0 specification introduces a new mixin named 'mix:etag' that defines a single 'jcr:etag' property designed to be used similarly to HTML entity tags (see http://www.ietf.org/rfc/rfc2616.txt section 3.11). The basic idea is that this mixin can be added to a node (with BINARY property values), and the 'jcr:etag' property will be automatically set to an opaque, implementation-determined value. This 'jcr:etag' property will then be automatically changed when any of the BINARY properties are changed. Clients can then check this 'jcr:etag' property value to know if the node has changed since the last time the client saw the node.

This appears to be a required feature of JCR 2.0.