My guess is this is "non-formula" in that there is a lookup instead of evaluating a function that uses the plaintext contents. Excessively simple example of Caesar cipher:

$plaintext = 'Attack at dawn!';
# formula - very simple encoding function
foreach $chr (split //,$plaintext) {
  $ciphertext .= chr(ord($chr)+1)
}
print $ciphertext,"\n";
$ciphertext = '';


# non-formula
# only need to do this next step once, then store hash somewhere to us
+e when encoding
foreach $letter ('A'..'Z','a'..'z',' ','!') {
  $shifted{$letter} = chr(ord($letter)+1)
}

foreach $chr(split //,$plaintext) {
  $ciphertext .= $shifted{$chr}
}
print $ciphertext, "\n";
[download]