This value configures a custom page indicator text. It consists in a pattern that lets you precisely configure which value to display and how. There are three values that can be used in the expression:
Each value need to be followed by a number format, as defined here : https://developer.android.com/reference/java/text/DecimalFormat.html.
Let's take an example: %1$1.2f. This can be split in two parts %1$ and 1.2f. %1$ is the x position. 1.2f is the number format where 1 stands for one digit before the comma, 2 means to display 2 digits after the comma and f means to display a floating point number.