Features used to implement the example

• Set a field as a function of other fields

Example: Setting a field's default value depending on another field

Can the Jira Workflow Toolbox plugin set a default value based on the contents of a field? For example, I have a custom field called Billable which is a checkbox. I would like to set this field to 'selected' if the issue type is 'Change Request' and set this field to 'unselected' if the issue type is 'Defect'.

It's possible to do it easily using post-function Set a field as a function of other fields, but there is something I don't understand in your example: I can't find any type "Checkbox" for custom fields in Jira. There is a custom field type: "Multicheckbox" but it doesn't seem to be suitable for your example since if you have options "Yes" and "No" you could check both options at the same time. I find more suitable for your example custom field types "Radio button" or "Select List". 

Let's suppose a field "Billable" of type "Select List" with two options "Yes" and "No". In this case you need to add to transition "Create Issue" a post-function Set a field as a function of other fields configured as shown n the following screenshot:

Text to be parsed is:

(New Feature)Yes

(.*)No

Once configured, post-functions configuration for transition "Create Issue" looks like this: