question

Correct - all our service API calls are REST at the base, with the SDKs providing wrappers around that to provide HTTP and JSON functionality. So the REST documentation applies to all our SDKs, and the call patterns are the same throughout - all the SDKs have the same method names and parameters.

For statistics specifically:

UpdatePlayerStatistics: This is for updating statistics which belong to the players account directly, and is the most common use case.

UpdateCharacterStatistics: This is for updating statistics specifically associated with a Character, which is a sub-element of the player account. Think of them as being like character slots in an MMO - they have distinct names, stats, and inventory, but they belong to the player account.

UpdateUserStatistics: This is our older API call, for updating stats for a player. It is replaced by UpdatePlayerStatistics, and will be deprecated soon.

Now since you're using Unity, I would recommend checking our PlayFab-Samples repository, as it shows a number of "recipes" for how to do things in Unity: https://github.com/PlayFab/PlayFab-Samples.

For example, if you have a look at the way UpdateUserStatistics is called (and UpdatePlayerStatistics follows the same model), you'll see that the parameter for input to this function is UserStatistics - a string, int Dictionary. This is because you can pass in many statistics in a single call.

So to make your call, you would actually want to do something like this:

UpdatePlayerStatisticsRequest request = new UpdatePlayerStatisticsRequest();
request.UserStatistics = new Dictionary<string, int>();
request.UserStatistics["Kill"] = score;
PlayFabClientAPI.UpdateUserStatistics(updateRequest, UpdateUserStatsCallback, SharedErrorCallback);

For your other questions:

No, statistics currently cannot be deleted. We will be updating to provide a way to do this later this quarter.

And yes, as stated in the documentation for the statistics update calls, all statistics are leaderboards in PlayFab - that's specifically the design intent, to provide a simple way to create leaderboards. And yes, each statistic can be configured to reset on a pre-defined interval. If your design is to track a score for players, and have it reset on a daily, weekly, and monthly basis, then yes, you would need three separate statistics (and so, leaderboards).