STAK (Stichting Administratiekantoor): This is a Dutch foundation used to issue depositary receipts for shares, a common form of employee participation in the Netherlands. It allows employees to benefit from the financial gain of share ownership without directly owning shares or having voting rights.

Share Plan Certificates: These are essentially physical or digital certificates that represent ownership of shares in a company. They're used less frequently today due to the prevalence of electronic records, but some companies still use them, especially for employee share ownership plans. Each certificate usually shows the number of shares owned, the date, a corporate seal, and signatures.

Restricted Stock Units (RSUs): RSUs are company shares that are promised to an employee through an employment agreement but are not fully 'owned' by the employee until a certain set of conditions are met (vesting conditions). These conditions can be based on length of employment or performance milestones. Unlike stock options, RSUs maintain some value even if the company stock price declines.

409A Valuation: This isn't a type of stock option, but rather a valuation requirement that applies to stock options. According to Section 409A of the U.S. Internal Revenue Code, companies must determine a fair market value of their common stock (i.e., the price at which they grant stock options) using reasonable valuation methods. This valuation is often performed by a third-party specialist and is important for setting the strike price of stock options.

EMI (Enterprise Management Incentives): This is a type of stock option scheme in the UK. EMI schemes are designed to help small, higher-risk companies recruit and retain employees by providing them with a tax-advantaged way to own shares in the company. EMI options are often granted at a discount to the fair market value and can offer favorable tax benefits.

Incentive Stock Options (ISOs) and Non-Qualified Stock Options (NSOs): These are two common types of stock options in the U.S. ISOs offer tax advantages and are only available to employees, while NSOs can be issued to employees, consultants, board members, etc. The tax implications differ between ISOs and NSOs, particularly at the time of exercise and sale.