The Casio FX-991ES PLUS-2 is a highly capable scientific calculator designed to assist students, engineers, and professionals in performing complex mathematical calculations quickly and accurately. It offers a wide range of features and functionalities to accommodate various mathematical needs. Here's a detailed description of its key features:

1. Natural Textbook Display: The calculator displays mathematical expressions and results just as they appear in textbooks, making it easier for users to understand and verify their calculations.
2. Multi-Replay Function: This feature allows users to backtrack through calculations step by step, edit expressions, and recalculate results. It's helpful for reviewing and correcting calculations.
3. High-Resolution LCD Display: The calculator features a high-resolution screen that ensures clear visibility of numbers, symbols, and mathematical expressions, even in dimly lit environments.
4. Advanced Mathematical Functions: It supports a wide range of mathematical functions including basic arithmetic operations, fractions, roots, powers, trigonometric functions (sine, cosine, tangent), logarithmic and exponential functions, complex numbers, and more.
5. Matrix and Vector Calculations: The calculator is capable of performing matrix and vector operations such as addition, subtraction, multiplication, inversion, determinant calculation, and solving systems of linear equations.
6. Integration and Differentiation: It offers numerical integration and differentiation capabilities, allowing users to find definite and indefinite integrals as well as derivatives of functions.
7. Equation Solver: The calculator can solve various types of equations, including linear, quadratic, cubic, and simultaneous equations. It also supports inequalities and polynomial equations.
8. Statistical Analysis: It includes a comprehensive set of statistical functions for analyzing data, such as mean, standard deviation, regression analysis, probability distributions, and more.
9. Engineering Notation and Unit Conversion: The calculator supports engineering notation for representing large and small numbers conveniently. It also facilitates unit conversions for different measurement units.
10. Battery-Powered: The calculator typically operates on battery power, providing portability and convenience for users who need to perform calculations on the go.