Difference between $,$$,$$$ in Informatica

Difference between $,$$,$$$ in Informatica

1. $ refers
These are the system variables/Session Parameters like $Bad file,$input 
file, $output file, $DB connection,$source,$target etc..

2.$$  referes
User defined variables/Mapping Parameters like $$State,$$Time, $$Entity, $$Business_Date, $$SRC,etc.

3.$$$ refers
System Parameters like $$$SessStartTime

$$$SessStartTime returns the initial system date value on 
the machine hosting the PowerCenter Server when the server 
initializes a session. $$$SessStartTime returns the session 
start time as a string value. The format of the string 
depends on the database you are using.

$$$SessStartTime returns the session start time as a string value --> The format of the string depends on the database you are using

SESSSTARTTIME is stored as a transformation date/time datatype value.
SESSSTARTTIME is used in the internal Informatica expression language, e.g. in an expression, aggregator, filter, router, etc.