Specs of the product are essential in in the Design Specification Document to ensure that testing and development can be properly executed. The specs of the product must be properly documented to allow for product development in the future. In addition, any product will have a range of error in development accounting for slight variance in size and shape. This must be accounted for prior, therefore when it is sent out to be tested on, the results can be verified. As for determining the specs, that largely depends on the product/device you are developing. In terms of the acceptable range of variance accounted for on your documentation, it is required that all means of failure and error be accounted for. For example, to submit the specs for testing and production of a device that will be 3D printed on a Mark Forged printer, it could be necessary to account for a dimensional inaccuracy of up to 125 microns.