Here is a list with all template variables that can be used in the respective template files:

You can use the language variables in any template file (globally). The variables from "tpl/language.php" are prefixed with "L_". Examples: {L_NAME}, {L_EMAIL}, etc.

{U_POST_FORM} - the URL address to which our form posts data.

Subject group:
{SUBJECT_STAR} - this variable is empty upon form submission error. Otherwise it shows a star (as defined in $g['lang']['client_star'] in "tpl/language.php").
{F_SUBJECT} - initially this variable is empty and represents the "value" of the SUBJECT <input> field. When the user submits something and the input has to be resubmitted (due to bad input from the user), this variable contains the original user input for the SUBJECT field.
{F_SUBJECT_LENGTH} - a number which shows the maximum length of the SUBJECT field.

There are 4 such groups more - for the fields Name, E-mail, Location, and Homepage.
The template variable names are formed in the exactly same way but "SUBJECT" is to be replaced with the following text respestively:
NAME
EMAIL
LOCATION
HOMEPAGE

Comment group:
{COMMENT_STAR} - the same as {SUBJECT_STAR} respectively for the comment field.
{F_COMMENT} - the same as {F_SUBJECT} respectively for the comment field.

Image protection group:
{U_SECURITY_IMG} - URL address of the code image.

Hidden <input> fields group:
{HASH} - security hash. It must be submitted for the user input to be accepted (e.g. login, comments).