The options view allows the user to set the common calculator options.
The number displayed is the current value of the x register. This number will update for changes in number format and digits.
Number Format: Choose whether or not to have thousands separators or scientific number format.
When Fixed Digits is selected, digits
are to the right of the decimal.
When Significant is selected, the number of digits
are the total significant digits.
The following table shows how different values are shown with Digits set to 4 and Number Format set to Separators.
Value  Fixed Digits = 4  Significant Digits = 4 

1234.56789

1,234.5679

1,235

0.00123456

0.0012

0.001235

Significant digits are typically used for Statistics and Scientific
applications.
Minimum Digits 0 or MAX applies to the fractional part
of the significant option. So 2.0000 would be displayed as 2 when this
option is set to 0.
12 or more significant digits with 0 max digits is similar
to the "ALL" number format on some calculators.
When the Fractions option is selected the parameters for fractions are displayed.
The following table shows how 3.14159 is displayed with a Max Denominator of 16 for the Most Precise and Fixed Denominator Option.
Fixed Denominator NO  Fixed Denominator YES  

Most Precise NO

3 1/8 (picks a factor of the max)

3 2/16 (alway uses the max)

Most Precise YES

3 1/7 (most precise <= the max)

Can only be set when Most Precise = NO

The Rows to Display are the stack rows in the Focused or Formula calculators in CHN or RPN Calculation Mode. You can also use the swipe up/down gesture to increase and decrease the rows displayed.
Display Stack Labels: This will toggle the x, y, z, t labels to the right of the stack display. There is no stack display when one row is displayed. This only applies when the Calculation Mode is set to CHN or RPN.
Calculation Mode can be set to chain (CHN), Reverse Polish Notation (RPN) or Algebraic (ALG).
Turning on Key Clicks applies to the calculators. You adjust the volume of your key clicks using your device volume controls.
The Haptic Feedback option is for devices that support haptic feedback.
Enabling Display Key Previews will display the value for any STO/RCL keys. This includes the top row of keys for Formula Calculators and the TVM calculators. The key preview number format uses a significant digits format to display the values in the smaller space of the keys.
Button Text Size allows you to adjust the size of the text on calculator buttons for the device. This is in addition to the iOS Dynamic Type settings
Some formatting options can also be set using the Number Formatting Keys.