I found this on subscription of RB433AH on routerboard.com website:
A microSD card gives you the ability to add more storage for Webproxy cache, logs, or Metarouter virtual machines.
My question is:
How can I use microSD for my metarouter virtual machine?
I want to have matarouter with OpenWRT for asterisk, and i want to run this metarouter on microSD instead system disk.
Thanks for answer.
currently you cannot have running metarouter on microSD card.
OK, but why is this information on your Webside? 
A microSD card gives you the ability to add more storage for Webproxy cache, logs, or Metarouter virtual machines.
more probably author of those lines meant - images and backup/export of guest configuration.
MetaROUTER running from microSDHC or USB Flash drive provide xGB data for web-server, mail-server and other applications.
maybe someday this will be made a reality, but for now - it is just a nice feature request for some time already.