__CSVRead() function
The CSVRead function returns a string from a CSV file.
When a filename is first encountered, the file is opened and read into an internal array. If a blank line is detected, this is treated as end of file – this allows trailing comments to be used.
All subsequent references to the same file name use the same internal array.
Each thread has its own internal pointer to its current row in the file array. When a thread first refers to the file it will be allocated the next free row in the array, so each thread will access a different row from all other threads.
eg: You need to test login to an app with different user login credentials for different threads.
__Stringfromfile()
The StringFromFile function can be used to read strings from a text file. Each time it is called it reads the next line from the file. All threads share the same instance, so different threads will get data from different lines. When the end of the file is reached, it will start reading again from the beginning, unless the maximum loop count has been reached.
Note: If there are multiple references to the function in a test script, each will open the file independently, even if the file names are the same. So it is advisable to use different file name across same scripts otherwise the output will be unpredictable.
Eg: You need to add different products to a shopping cart for the same user. In this case you will be using CSVRead to read user credentials and Stringfromfile to read product info that need to be added to each user.
So in a nutshell, Difference between __CSVRead and __Stringfromfile function is that Stringfromfile should be used when different data are required in a loop for same thread and CSVRead should be used when different data are required for different threads(users).