Explanation
https://docs.splunk.com/Documentation/Splunk/8.1.0/SearchReference/ConversionFunctions#tostring.28X.2CY.

The transaction command is a search command that creates a single event from a group of events that share
some common characteristics. The transaction command can group events based on fields, time, or both. The
transaction command can also create some additional fields for each transaction, such
as duration, eventcount, startime, etc. The transaction command does not group a set of transactions based on
time, but rather groups a set of events into a transaction based on time. The transaction command does not
separate two events based on one or more values, but rather joins multiple events based on one or more values.
The transaction command does not return the number of credit card transactions found in the event logs, but
rather creates transactions from the events that match the search criteria.

Reference:
https://docs.splunk.com/Documentation/SplunkLight/7.3.5/GettingStarted/Designdatamodelobjects