Sounding the same and being spelled very similarly, these two words are commonly confused in business writing. Keeping the two words straight will ensure that others clearly understand your work and the meaning is sound from a legal perspective.A principle refers to a rule or a standard and is always used as a noun. "The key principle of writing is to ensure you convey your message effectively," would be a way in which you would use this spelling of the word.
A principal can have a couple of meanings and is the one used more commonly in business documents. In once sense a principal refers to a senior individual, the principal of a school or a principal in a business (which is synonymous with partner in many cases). In a legal context the principal can also refer to an individual upon whose behalf an agent is acting, a lawyer can act on behalf of a principal when carrying out obligations for them.
Keeping principal vs. principle straight can ensure that the meaning of your writing is always conveyed effectively and you don't accidentally say the wrong thing.