Exactly, multiplex PCR means several primer pairs amplifiy several genomic targets. The difficulty in multiplex PCR lies within cross-hybridizations between different primers, especially when there are many reactions running in parallel.

http://www.premierbiosoft.com/tech_notes/multiplex-pcr.html

In Multiplex PCR, several Primers can amplify different genomic target in single PCR reaction and PCR program. Designing of PCR primers to be used in Multiplex mode is quite difficult and precautions should be taken in computation of these primers to avoid cross hybridizations.

Thanks for this clarify.