The instruction not to make clothing that mixes 'woolen and linen' threads means, in the internal sense, that people's spiritual states of good and truth should not be mixed up with one another. See the explanation of this in Arcana Coelestia 10669.

This is an interesting rule, because in many places, New Christian doctrine makes it very clear that there needs to be a marriage of good and truth in each of us. And yet, in heaven, there's a spiritual kingdom, and a celestial kingdom. Both are heavenly, but the people's loves in the spiritual kingdom are different than those of people in the celestial kingdom. They have different approaches to life, and that's by design, and the distinction is useful.

(References: Deuteronomy 22:11)