Typically to convert from UOM1 to UOM2 you perform the following key sequence:
Enter the UOM1 value
Tap the UOM of the value.
Now to convert the value:
Tap the UOM of UOM2
Convert 100 yards to meters using the Length conversion calculator.
The display will show 91.44.
Anytime you are unsure of the abbreviations on the labels, use the long press gesture to display the hint for that key.
Following the steps in General Key Details will allow you to see the conversion factors used.
If you need another UOM added, copy and paste an existing key (both the STO and RCL key ) and follow the instructions in Modify or Create Keys. These are relatively easy key formulas to modify. Many conversion factors can be found at the Wikipedia reference below.
The CalcFxC conversion calculators convert the UOM to the standard SI UOM when you press STO. Then when you press RCL, the standard SI UOM is converted to the selected UOM. Using this system it only takes two keys for each UOM. For calculators that have two key per conversion (yards to meters and meters to yards), it takes N^2-N keys for N-UOMs. So it would take 182 keys to provide the 14 length UOM conversions using a separate key for each conversion.
But since you are using CalcFxC, you can easily create a key to perform a conversion you use a lot.