Scores are submitted anonymously by the players of your game and aggregate information about those scores are stored.
There is no need for the player to be logged into a service such as Google Play Services or Game Center.
The collective leaderboard system keeps track of the best and worst score of the day, along with the average and number of scores.
Using this aggregate data, it allows you to give contextual information to the player regarding their score compared to others such as
"You are the 3rd person to score today!"
"You are in the top 15% of players worldwide!"
"You got the best score this week!".
Each day the credits charged will be equal to the number of submitted scores divided by 1000 rounded up to the nearest whole number.
For example if you had 1500 scores submitted on one day the charges would be
ceiling(1500 / 1000) = 2 credits.
How much do credits cost? See pricing