Each type of filter you select will be additive, effectively acting as an "AND WHERE" clause to the rest of your filters. For example, a search request with parameters for both entity_types and specialties like so:

{
  "specialties": [
    "Rehabilitation Hospital Unit"
  ],
  "entity_types": [
    "Facility"
  ]
}

will filter for practitioners that have a Entity Type of "Facility" AND a specific NUCC specialty of "Rehabilitation Hospital Unit".

However WITHIN a given filter type, multiple values will be unioned, effectively acting as an "OR WHERE" clause within that filter. So a search body of the following, with multiple specialties selected:

{
  "specialties": [
    "Rehabilitation Hospital Unit",
    "General Acute Care Hospital"
  ],
  "entity_types": [
    "Facility"
  ]
}

Will search for a practitioner with a Entity Type of "Facility", and a NUCC Specialty of "Rehabilitation Hospital Unit" OR "General Acute Care Hospital".