In old Drools, using a global variable to collect rule outcome is a popular use case.
e.g.

global java.util.List resultList;

rule Hello
  when
    $p : Person(age > 20)
  then
    resultList.add("Hello " + $p.getName());
end

Add an example for such a use case so that users can follow a recommended approach. Probably A) SingletonStore.

A) SingletonStore

https://github.com/tkobayas/my-kogito-examples/tree/master/hello-singleton-store-declare

declare Hello extends RuleUnitData
  result : SingletonStore<Result> = DataSource.createSingleton()
  messages : DataStore<String>
end

rule helloWorld
when
    $r : /result
    $m : /messages
then
    $r.setValue($m + " world");
end

query hello
  $r : /result
end

Concerns:

• Need to refer the variable at LHS (I think it's acceptable)
• Where should I insert the object firstly? (Currently, via POST payload)
• Has a minor issue in generating from "declare" (Filed https://issues.redhat.com/browse/KOGITO-2127)

B) Plain field in RuleUnitData

A plain field in RuleUnitData can work as a global.

public class LoanUnit implements RuleUnitData {
    private int maxAmount;

However, it has some issues. Probably this approach is not recommended even if the issues will be fixed.
https://github.com/tkobayas/my-kogito-examples/tree/master/hello-global

• If you use a List as a plain field in RuleUnitData, it's considered as a data source (See ReflectiveRuleUnitDescription.getUnitVarType()) so failed in compiling RuleUnitInstance class.

  2020-05-08 14:54:26,536 ERROR [io.qua.dep.dev.DevModeMain] (main) Failed to start Quarkus: java.lang.RuntimeException: io.quarkus.builder.BuildException: Build failure: Build failed due to errors
  	[error]: Build step org.kie.kogito.quarkus.deployment.KogitoAssetsProcessor#generateModel threw an exception: java.lang.IllegalStateException: src/main/java/org/kie/kogito/examples/HelloRuleUnitInstance.java (13:30) : cannot find symbol
    symbol:   method subscribe(org.kie.kogito.rules.units.EntryPointDataProcessor)
    location: interface java.util.List<java.lang.String>
  src/main/java/org/kie/kogito/examples/HelloRuleUnitInstance.java (13:30) : cannot find symbol
    symbol:   method subscribe(org.kie.kogito.rules.units.EntryPointDataProcessor)
    location: interface java.util.List<java.lang.String>
  	at org.kie.kogito.quarkus.deployment.KogitoAssetsProcessor.register(KogitoAssetsProcessor.java:314)
  	at org.kie.kogito.quarkus.deployment.KogitoAssetsProcessor.generateModel(KogitoAssetsProcessor.java:197)
  

• Cannot write a query for global. So cannot generate REST endpoint.