RTFM (see edits for links):
18.104.22.168.5 The Primer Filter
The ends of all processed reads are scanned for similarity to Genome Sequencer System Adaptor sequences. Any flows corresponding to Adaptor sequences are trimmed from the read. If a trimmed read is shorter than 84 flows (21 cycles, or approximately 50 nucleotides), it is rejected. The output metric for these failed reads is “numTrimmedTooShortPrimer”.
PS is it harmful to answer a negatively rated question!
PPS I think there is enough to answer the question in the question
Edit manual links (v2.3):
I actually think 2.0 was better laid out, but I can't find a link. Contact your roche rep and ask for a u/p to the Roche FTP site.