Customizations represent a significant risk to delivery of the design. A functional consultant has a face-to-face meeting/conference call where he hopes to communicate the reason for the design, and checks the understanding of the technical consultant.
These are the risks from a functional perspective:
How do I know that the technical consultant understands what I want?
How do I make sure the customization will choose the correct data set & update the appropriate data fields?
Have I explained the design so that its fully understood?
Have I recorded enough information in the functional design document?
An appreciation of technical knowledge mitigates these risks to a large degree. The functional consultant can indicate how to select the appropriate data set, what fields to update, and how to report on the outcome.
If necessary, the functional consultant can get involved in unit testing so that later testing phases are smoother, and the overall delivery time for the customization is shortened