When I was hired I was told my program area, but the hiring notification process will go from HR this year, not from the program. This means you will only receive an official offer, as far as I know no unofficial offers will be made this year. I'm not sure how it will work this year, but in previous years people were told their broad program area aka "communicable disease" or "maternal health".
Although your assigned program area will determine the specific knowledge you gain, the program is centered around the broad skills you learn, which are often applicable across programs. Many people spend a couple of years in a program they have less interest in during PHAP and then have opportunities to jump to what interests them when the program is over.
This is just speculation, but I'd suspect that there would be no problem turning down the placement and then applying next year. However if you accept and start the process and then back out later, you might burn some bridges. The agency is small and people remember.
