Proximity (px-core) is a generic fetch-and-cache engine with various extra capabilities like indexing. The Px-Core module is driven by Maven bindings (px-core-maven) to implement a Maven Proxy application behaviour.
Proximity is in function somewhere between http-proxy and proactive-mirror. Proximity is not HTTP Proxy. One of it's primary use is as Java web application to serve as maven proxy on our company's intranet. As for reducing outgoing traffic (caching central and other maven repos), aggregating more repositories (reducing project config) with acting as one logical repository and for publishing in-house and other external maven artifacts which are not uploadable to ibiblio (like commercial projects, J2EE Jars, etc...).