More clarification required

Hi Remi,
I am working on having a control chart for Defect density and I have a similar problem of having defect data with too many zeros.
What I understand from your reply is to plot non-zero points separately in control chart & For zero data points, use % of non-zero in G-chart. However I am not very clear with CTQ1(the % of non-zero part). Can you give more details on this ?
I am doing this as part of software process improvement in CMMI & not fully familar with six sigma concepts like CTQ. So I would appreciate if you can explain in simple terms.
Thanks
Sri