Using field method access is deprecated and will be removed
Recommend using the more complete Substitution variables with an explicit prefix.
Only use for transition variable access
These are the Jira internal field names and are needed if you want to access transition fields. Normally, this is only needed for accessing screen field values on a workflow validator. For post functions, the value is already in the issue variable.
Reference is JIRA 5.2 IssueFieldConstants.java