Thanks Scott.
The documentation at https://docs.pods.io/ is not that helpful in my case.
I used some PHP in the template, even if this is deprecated. It works for me.
<?php
? $nummer = $obj->display( 'nummer' );
? if($nummer && str_starts_with($nummer, 'PP')) {
? ? echo '<span class="fr" title="Nummer im Katalog 1: ' . $nummer . '">'. $nummer . '</span>';
? }
? else if($nummer) {
? ? echo '<span class="mi" title="Nummer im Katalog 2: ' . $nummer . '">'. $nummer . '</span>';
? }
?>