Answered Configuring external caching (APC, Memcache, etc)

You can cache whatever partitions of the MODX cache you want to in memcached, but be aware that if something is larger than 1Mb, it will not be cached by memcached unless you have specifically configured memcached to allow larger cache entry sizes.

The global cache configuration, determined by the cache_handler and cache_expires settings apply to all partitions by default, unless overridden by particular partitions. Each cache partition stores a particular part of the MODX cache and is identified by a key which identifies the cache provider instance responsible for that partition. The key for the cache partition responsible for Resources is called "resource", so any cache_resource_* Settings will override those set globally for that specific cache handler instance. Similarly, the Context Settings are stored in the context_settings partition, overridden by the cache_context_setting_* Settings.

More information on these can be found here.