The Substring process accepts as input a string, then outputs a portion of it as another string. This is useful if you need to take part of an input file name and use it as part of an output file name.
In the Properties window, “Start” defines the beginning of the substring (starting at 0 for the first letter) and “Count” defines the length of the substring. For example, if the input string is 04_12_2021_lake, “Start” is 6, and “Count” is 4, then the output string is 2021.