System Fields in ABAP

 All the below system fields belong to the data dictionary (DDIC) Structure - SYST.

SY-INDEX	for loop counting
SY-SUBRC	condition of previous statement
SY-FDPOS	position of substring in main string (works with SEARCH)
SY-ABCDE	a string of 26 characters
SY-ULINE	for horizontal lines of any length
SY-VLINE	for only one vertical character
SY-DATUM	for application server's date
SY-UZEIT	for application server's current time
SY-DATLO	for Presentation server's date
SY-TIMLO	for Presentation server's current time
SY-DBSYS	backend database name
SY-HOST	name of application server
SY-SYSID	name of the R/3 system
SY-OPSYS	name of application server's operating system
SY-MANDT	Logon client number
SY-UNAME	Logon User name
SY-LANGU	Logon language
SY-MSGID	message class name
SY-MSGNO	message number
SY-MSGTY	message type
SY-MSGV1	message variable 1
SY-MSGV2	message variable 2
SY-MSGV3	message variable 3
SY-MSGV4	message variable 4
SY-LSIND	secondary list index
SY-PAGNO	current page number
SY-LINSZ	current width of a line
SY-LINCT	current number of lines per page
SY-TABIX	index of internal table's record
SY-DBCNT	no. of records processed by database statement
SY-DYNNR	current screen number
SY-REPID	current program name
SY-TITLE	title of the program
SY-PFKEY	name of the current GUI status
SY-UCOMM	funtion code of the current pushbutton or menu option

Courtesy: ABAP Technical Documentation