In the Java Control Panel "Desktop Settings" tab there is an "add" button for finding "non-installed" jres to be added to deployment.properties as possible jre to be used by jnlp applications and applets.
When pointing to a modular jre (Java 9, Java 10, ...) this will not work.
(Although you can manually add such a JRE by adding all 9 jre.xxx.... properties to deployment.properties)

The reason for this is the mechanism used to check if it is a valid jre is to call java with args:
basic: 0: C:\cygwin\home\aherrick\jdk9\build\windows-x64\images\jdk\bin\javaw.exe
basic: 1: -classpath
basic: 2: C:\cygwin\home\aherrick\10\build\windows-x64\images\jre\lib\deploy.jar;C:\cygwin\home\aherrick\10\build\windows-x64\images\jre\lib\controlpanel.jar
basic: 3: com.sun.deploy.jcp.controller.JreLocator

there is no controlpanel.jar in the lib directory and deploy.jar does not have the class basic: 0: C:\cygwin\home\aherrick\jdk9\build\windows-x64\images\jdk\bin\javaw.exe
basic: 1: -classpath
basic: 2: C:\cygwin\home\aherrick\10\build\windows-x64\images\jre\lib\deploy.jar;C:\cygwin\home\aherrick\10\build\windows-x64\images\jre\lib\controlpanel.jar

This will only work for jres version 6,7,8 - the new control panel in jdk9 is in a different place.