Tyl has prebuilt functions that have a fixed functionality.
Tyl system functions features:
Throughout this guide all system functions in code boxes are in dark blue color.

We met the print system function in get started page. print function is used to print messages to the run box. print function has only one parameter, and that is object parameter.

Here's print function description:
print object
object parameter is self explanatory but nonetheless can be detailed as:

The object to print, such as string, number, list or record.

Any function has a return value. For regular functions, it is the value of their last executed statement, but system functions has their own inherent return value. System function return value will be written in the function description after the parameters, prepended by the arrow symbol ''.

Here's print description with return value:
print object → object

object return value, will be detailed as:

object parameter

Note that if the return value is one of the function parameters, it may not be detailed, and the return value will be the value of the parameter after the function has finished to be executed.

Here's full len system function description:
len var → length
Tyl variable
for string, returns string length, for list - the number of items in the list, and for record - the number of keys in the record mapping data.

Some system function parameters are optional, meaning, the function do not require them. For ex., the beep system function has two parameters, but optionally can be called without parameters. Another example is string.section function in string module that its length parameter can be omitted. So if a parmeter has default value or the description specifies that it can be omitted, than this is an optional parameter.

All system functions are in System Functions Reference