If you want to limit the available options for a specific facet, you can do this with a queryGroup. In this example I've limited the available categories. This example is meant for TYPO3 and EXT:solr

tx_solr_facet.ts

plugin.tx_solr {
	search {
		faceting {
			facets {
				main {
					label = Category
					field = category_stringM

					type = queryGroup
					queryGroup {
						cat1 {
							query = cat1
						}

						cat42 {
							query = cat42
						}

						cat999 {
							query = cat999
						}
					}
				}
			}
		}
	}
}

Further documentation can be found here:
http://forge.typo3.org/projects/extension-solr/wiki/Tx_solrsearch#facetingfacetsfacetNametype