If you select a Credit-based (On Demand) API Service you will be charged per record (using MarineTraffic Credits). Note that the cost of each record returned depends on the subject API Service and the parameters you set.
To make the distinction between records and Credits more clear, think of a record as a collection of response fields (as mentioned in the Documentation section of the corresponding API Service) of different data types, in fixed number and sequence.
For example, the following example contains two records (in .csv format):
MMSI, LAT, LON, SPEED, HEADING, COURSE, STATUS, TIMESTAMP
237955000,37.884910,24.734500,0,283,191,0,2016-09- 19T12:38:18
319738000,37.980610,24.395880,7,295,7,0,2016-09- 19T12:41:23