With Economy v2, wAssert.IsNull(e can use ETags to detect and mitigate concurrency issues. For example, when we load a player's inventory in the game client, we can save the associated ETag. When the player tries to buy an item, we can include the last ETag we received with the request. If our ETag is out of date (because some other operation that we're not aware of changed the player's inventory), the request fails; presumably, at this point we would then refresh the player's inventory and then try again.
However, when the request fails because the ETag is out of date, PlayFab just gives us back a generic 400 Bad Request error (the PlayFab error code is 1071 InvalidRequest).
This is essentially useless, as it isn't safe to assume that a 400/1071 error always means that our ETag is out of date. There must be a specific error code (e.g. InvalidETag) so that we know how to appropriately handle the error.