One transaction of Data Output is counted for each register read from a data bucket. Learn more about Getting Data.
You need to define the Data Output limit per month for each Profile where your applications are running.
Here is one example: a device reads one variable resulting in a response of 100 registers at every 10 minutes (100 temperature data points), the operations will result in 100 * 6 = 600 transactions per hour, which can then be converted by multiplying by 720 to result in 600 * 720 = 432,000 transactions/mo.
If the limit is exceeded, any attempt to read data (GET) will be denied for the rest of the month.
Accessing data from the dashboards doesn't count against the Data Output limit, even if data is downloaded in .csv format from the widgets. But, the system will count data output when using the function 'Export Data' from the bucket.