The ReactOS Calc provides some buttons for typing numbers and expressions.
However, some of them are available only into Standard or Scientific modes, or when the calculator is in a specific state.
Digit buttons: they are used for entering digits into the display.
Hexadecimal buttons: they are used for completing an hex number.
For using these buttons, the calculator must be configured into scientific mode and the [HEX] state must be selected.
or 
Decimal point: used for starting the non integer part of a decimal number.
Change sign: if it's pressed after a calculation or when entering the operands, it will change the sign of the number shown into the output display.
If the displayed number has been entered manually and it isn't a result of a calculation, you can delete the last digit by pressing this button.
This button removes an incorrect number from the output display. All pending operations are preserved.
It clears the output display and all pending operations.