Signature updates are cumulative and increment by one with each new release (for example, S145, S146, S147). Signature updates include every signature since the initial signature release (S1) in addition to the new signatures being released. Signature updates require the minimum version listed in the filename.
To install the most recent signature update, you must have the most recent minor version. Service packs are dependent on the most recent minor version, which is dependent on the most recent major version.
http://www.cisco.com/en/US/products/hw/vpndevc/ps4077/products_configuration_guide_chapter09186a00804cf503.html