Fee Billing Rule - Strange Behaviour

Fee billing rule also has some strange behavior. Will update this post once i find the right reasons for this behavior. But for now just wanted to note this behavior for future reference.

Fee billing rule can only be defined if there is an existing billing rule in place, which is other than fee or else you get this error message. This is a standard behavior, which is ok and accepted.

Error message when trying to create a Fee billing rule, without having any other billing rule created up-front.

Now that i have created a progress billing rule, i can then create a fee billing rule and can add the progress billing rule into it.

Progress billing rule added to the Fee billing rule, with fee percentage specified

 Now if a new project is created for the same project contract and if a new progress billing rule is created for the new project and if the user wishes to add the newly created billing rule into the already created Fee billing rule, then the system throws this error. This is what i found strange and also could not understand the error message and the logic behind it.

Error message highlighted, which states that Fee line can only reference a Fee line with a lower contact line number

There are two ways in which this can be handled.

First way is to add the project in the already created billing rule.

Adding a project to the already created billing rule

Second is to create a new fee billing rule and then add the new project into it.

New fee billing rule and adding a project to it

Also one last thing is, if the user tries to add a fee billing rule to the 'Selected items' in the fee billing rule, then the system complains and throws this error message. This is a correct behavior.

Error message when trying to add a fee billing rule to a fee billing rule

Thanks
Sarang