All searches have a security filter, which always has a securityType attribute, with one of several possible security types as its value.
A security type determines what level of security is applied to a given search. I.e. a security type asks the question "does the user have the appropriate rights to see the result of the search?".
There are many different security types, but the vast majority of those will not be described in this page. This page only documents two security types: "ItemSecurityReadAccess" and "ItemSecurityReadAccessOnlyPublic".
ItemSecurityReadAccess
Filters out any assets, which the current user does not have the rights to read.
ItemSecurityReadAccessOnlyPublic
The same as ItemSecurityReadAccess, but also filters out any assets, which are not public.