From Xojo Documentation
A a decimal number that holds 15 digits to the left of the decimal point and 4 digits to the right. The default value of a Currency is 0.0.
Currency can contain values from -922337203685477.5808 to 922337203685477.5807. It uses 8 bytes.
Currency is a 64-bit fixed-point decimal. This means that Currency does not have some of the rounding issues that are inherent in floating-point numbers stored in Double.
Use Currency instead of Double when storing monetary values. Since Double is a standard double-precision floating-point number, it is unable to store some specific decimal values. The Currency type avoids these issues, which can be particularly important with rounding and when dealing with money.
Assign a value to a currency variable:
Double data type