Can one fabric be best across every frequency?
Not necessarily. A material can perform differently across low, mid and high frequency bands.
Why are openings more critical at higher frequency?
As wavelength becomes shorter, smaller openings can become more significant leakage paths.
What should buyers request?
Shielding data across the actual frequency range of interest, not only one maximum value.
Project note: Use supplier data as a starting point. Validate the finished assembly at the relevant frequencies and environmental conditions.