X is Check, B is bet, C is call, R is raise, F is fold.
In format_number the 2 is number of decimals, though if you want to see just 1 in the HUD you can override that value in the Configure HUD section (after adding the stat, see the properties box).
From the
Tutorial: Using Custom Statistics & Reports:
format_number( value, decimals, commas, color )
* value - the value you want to display, typically copied from the Value Expression
* decimals - the number of decimal places to display
* commas - true if you want commas separators for thousands (e.g. 1,000 instead of 1000)
* color - true if you want positive values to be green and negative to be red. If you want the stat to be default color or if you want to define your own color ranges then you should set this to false.