The Request Parameters API allows creating parameter sets that can override or take the place of parameters defined in solrconfig.xml. The parameter sets defined with this API can be used in requests to Solr, or referenced directly in solrconfig.xml request handler definitions.
It is really another endpoint of the Config API instead of a separate API, and has distinct commands. It does not replace or modify any sections of solrconfig.xml, but instead provides another approach to handling parameters used in requests. It behaves in the same way as the Config API, by storing parameters in another file that will be used at runtime. In this case, the parameters are stored in a file named params.json. This file is kept in ZooKeeper or in the conf directory of a standalone Solr instance.
https://cwiki.apache.org/confluence/display/solr/Request+Parameters+API