UDM_PARAM_VARDIR - specifies a custom path to directory where indexer stores data when using built-in database and in cache mode.
UDM_PARAM_ISPELL_PREFIXES - Possible values: UDM_PREFIXES_ENABLED and UDM_PREFIXES_DISABLED, that respectively enable or disable using prefixes. E.g. if a word "tested" is in search query, also words like "test", "testing", etc. Only suffixes are supported by default. Prefixes usually change word meanings, for example if somebody is searching for the word "tested" one hardly wants "untested" to be found. Prefixes support may also be found useful for site's spelling checking purposes. In order to enable ispell, you have to load ispell data with udm_load_ispell_data().