Those variables will be two strings. For this demo those will be space-separated lists of items and fields. By the way the Client doesn't know about this and will interpret them as a group and a schema name (i.e. the Client doesn't know how many items/fields we are subscribing to).
on the server side the getItems method of MetadataProvider translates the received string in a list of items. In this example we split the string on the space character, but you may use any string as group and translate in as many items as you want on the metadataadapter.