Topic on Manual talk:$wgMainStash

By reading the Details page, it seems that $wgMainStash make a local fast accessing copy of shared cache. Is this true?

Should I set it to redis or CACHE_ACCEL? Does $wgMainStash only work when WANCache is used?

Is this setting correct if memcached is installed?

$wgMainStash = CACHE_MEMCACHED;