How can we assign a boolean value to null

Hey,

a!save( target, value)

In interface saveInto parameters, updates the target with the given value. Use a!save for each item that you want to modify or alter in a saveInto parameter. This function has no effect when called outside of a component (saveInto parameter).

According to your question, you can give this a try based on my understanding:

If outageEligible is false, save null in the outage rule input; otherwise, not

if(or(ri!outageEligible)=false,a!save(ri!outage,null),{})

In your code, you are comparing a boolean with a boolean. 

Instead, it could be further simplified as 

if(
  or(ri!outageEligible),
  {},
  a!save(ri!outage, null)
)

if(
  ri!outageEligible,
  {},
  a!save(ri!outage, null)
)

if(
  ri!outageEligible,
  {},
  a!save(ri!outage, null)
)

or() with booleans is used to convert null into false.

if() treats null just like false already.

I wish every function in Appian was designed this way, but sadly it varies. So this is a standard practice I use every time when dealing with booleans. 

Anything other than true (including null) evaluates else block.