Volatile Functions

A small number of the worksheet functions can be described as "volatile" functions.
This means that these functions constantly update when any changes are made to the worksheet.
If any of these functions are included in your array formulas and they are used with references to large cell ranges then the worksheet will not be very efficient.
Volatile functions will not update when calculation is set to manual.
The following is a list of volatile functions.

CELL	The number or text string indicating information about a cell.
INDIRECT	The text string of the contents of a given cell reference.
INFO	The text string returning useful information about the environment.
NOW	The date serial number of the current system date and time.
OFFSET	The cell value which is an offset from a given cell reference.
RAND	The random number between 0 and 1.
RANDBETWEEN	The random number between two specified numbers (inclusive).
RANDARRAY	The array of random numbers between 0 and 1.
TODAY	The serial number representing today's date.

CELL The number or text string indicating information about a cell.
INDIRECT The text string of the contents of a given cell reference.
INFO The text string returning useful information about the environment.
NOW The date serial number of the current system date and time.
OFFSET The cell value which is an offset from a given cell reference.
RAND The random number between 0 and 1.
RANDBETWEEN The random number between two specified numbers (inclusive).
RANDARRAY The array of random numbers between 0 and 1.
TODAY The serial number representing today's date.

CELL

This depends on the arguments

INFO

This depends on the arguments

SUMIF

SUMIF(A1:A4,">0",B1) is volatile
but
SUMIF(A1:A4,">0",B1:B4") is not volatile

INDEX

This is not volatile despite some documentation from Microsoft that says it is.

ROWS

This is not volatile despite some documentation from Microsoft that says it is.

COLUMNS

This is not volatile despite some documentation from Microsoft that says it is.

AREAS

This is not volatile despite some documentation from Microsoft that says it is.