By default UCF searches for a minimum version of JRE on the client machine. If minimum version is not found, UCF tries to install its own JRE on to the client machine.
Even though UCF can install its own JRE, the browser should have JRE to initiate/launch the java applet.
User can install different JREs on client machine and UCF should work.
With D6 the certified JRE is 1.5 (update 11). With D5.3 Sp6 the certified JRE is 1.6.
Even though the certified JRE version for D6 is 1.5, UCF will work for JRE 1.6.
For the list of JREs Vs Browser support for UCF, please refer to EMC technical Material