dynamic values in fnd messages in R12.1.3

Hi All,

How to use dynamic values in fnd messages, like the one used below:

We have a message GL_BIS_REVPMF_ABOVE_SUBJECT with current message text as :

(&PERIOD_NAME) Actual revenue exceeds planned revenue (for dimension levels &DIM1_NAME and &DIM2_NAME): (Actual: &ACTUAL, Planned: &TARGET)

Now I need to understand how system picks these dynamic values i.e value for &PERIOD_NAME. where does it look for this value. How this whole setup works in case I want to create a custom messages with my own dynamic values.

Please suggest. Thanks in advance.