What type of itemization/documentation is an employer required to provide to sales people paid variable compensation?

There is no requirement to provide documentation in this country (US). You are free to provide additional compensation whenever you’d like, based on whatever criteria you establish (or change). However, to maximize the motivational value of your plans, it is a good idea for both the employer and the employee to see the plans as a serious commitment by the company, and to have a written document showing exactly how it will work (measures, amounts, frequency, handling of disputes, eligibility, etc.). If you have such a document, that is in the form of a legally binding plan document, then you will also need to protect the company with language about management discretion, employee termination, when an amount is considered earned, etc. And you will probably want to run it by your legal counsel.

There are companies that successfully manage their sales forces without written plan documents. Typically their plans are very straightforward, and they don’t have very large sales forces. But the norm (and the best practice in most instances) is to have a good plan document that makes the “rules of the game” clear and serves as a helpful reference for all involved.