9.99% is usually a **regulatory and control threshold marker**.

Why that number is used:
- It keeps the LP **below 10%**, a level that often triggers extra scrutiny, filings, beneficial ownership issues, sanctions analysis, tax, ERISA, or other compliance concerns depending on context.
- In this clause, the goal is not to punish the LP, but to **reduce a Material Adverse Effect** while preserving as much participation as possible.
- Using **9.99%** is a common legal drafting convention because it is clearly under 10%, with a small buffer.

So the number is typically about **avoiding threshold-based consequences**, not because 9.99% has special economic meaning.

References:
- https://decilehub.com/base/1-general_questions/20774-what-is-the-cornerstone-start-agreement
- https://decilehub.com/base/1-general_questions/54353-how-does-the-gp-s-discretion-to-force-an-lp-s-withdrawal-due-to-a-material-adverse-effect-section-5-5-create-a-power-imbalance