I am wondering why we set critical and operational limits for metal detection within our HACCP plans if it is identified as a CCP, wouldn't we want to state that the critical limit is no metal? I am just slightly confused on this as we were asked if metal under those limits set is acceptable for a finished product. I am in a meat processing facility goverened by the USDA.
Thank you for your help!

Hi Elsa,
Just to hopefully summarise/expand the previous posts a little. Details are in various previous threads here.
Any specific limits usually have their own caveats , eg related to target consumer of the particular food category involved (ie risk assessment).
The concept of a (MD) zero limit CCP is obviously wishful thinking from a practical POV. Nonetheless it is a logical, Politically unchallengeable, "Official" response for the uninitiated.
if a CCP is involved some "typical" Regulatory published " limits" are -
USFDA - 7mm (!!!), USDA (FSIS) used to be 2mm, now I believe they have standardised to (perhaps reluctantly) agree with USFDA. Some US Industries may likely go their own merry way, eg "Best Practice", eg Baking Industry.
Canada - 2mm (non-meat IIRC)
A few European Countries - 2mm
Globally many countries have no specific limit and implement typically from Best Practice or Machine Operational Limit, ie approx LOD (limit of detection). Or copy other Countries
Operational limits are typically set outside critical limits so as to avoid failure of the critical limit. This then minimizes headaches such as activating / documenting corrective actions. It's as simple as that. 
PS -- some of the above numbers may not be entirely aligned to previous posts. If so i suggest some further searching.
PPS - just to note that Operational Limit has a more specific meaning in a FSSC22000 context but afaik this is not yr area of interest.
P3S - in addition to the legal interpretation, and as noted above, the practical crunch often comes when you need to validate the sensitivity of the MD.