What are "variables" and how are they different to fields?

In short, variables in ServiceNow are arbitrary fields on something that can take many forms and aren't tied to columns or fields in a table.

Typically, they are stored in a variable table of sorts in a key-value layout of "this variable has this value".

This is different to columns or fields in ServiceNow which are firmly-defined columns on a database table.

Almost all kinds of variable definitions extend from the parent "Variable" [var_dictionary] table.

Here's some examples of the differences between fields and variables.

Fields and variables on a requested item

Fields on a record

Variables in a list

What variables belong to which requested item

Catalog variables

There are 2 main areas for variables in catalog items:

  • The variables configured for the catalog items and variable sets. They form a "template" of sorts.
  • The variable instances & values themselves for the requested items. These are copies of the variable "templates" configured on the catalog items.

This method of making copies of the variables whenever they are used prevents variables from being broken on existing catalog requests whenever changes are made to the catalog items.

This includes formatting variables, including section starts & ends, and annotations.

Here's a diagram of the most common kinds of catalog variables.

Diagram of catalog variables

Here's a list of all of the tables involved in catalog variables that I could find.

Table Notes
Variable
[item_option_new]		 Extends "Question" [question]
The variable as it is configured on the Catalog Item / Variable Set.
Options
[sc_item_option]		 The variables and values for a request.
Producer Set
[item_option_new_producer_set]		 Extends "Variable Set" [item_option_new_set]
Same as Options [sc_item_option] but for record producers.
Variable ownership
[sc_item_option_mtom]		 A many-to-many table linking requested items [sc_req_item] to variables [sc_item_option].
Variables
[sc_item_variables_task]		 Which variables are visible on what catalog tasks.
Multi Row Question Answers
[sc_multi_row_question_answer]		 Appears to be a many-to-many table for multi-row variable sets, including what the question was, what set it's for, and the answer / value.
Variable Set
[item_option_new_set]		 Definitions for variable sets.
Item Options
[sc_cat_item_option]		 Appears unused.
Variable Category
[item_option_category]		 Appears unused.
Variable Layout
[item_option_layout]		 Extends "Question" [question]
Appears unused.

Questions

Underneath a large portion of variable functionality is something called "Questions". Some of the variable tables extend from "Questions" functionality, but not all of them.

Table Notes
Questions
[questions]		 The definition of some variables / questions.
Question Answers
[question_answer]		 The instances and values of some kinds of variables, including the below. I suspect that records created by record producers have the variables saved in this table.
Variables on CSM cases
[sn_customerservice_case]
Variables on Incidents
[incident]
Variables on Standard Change Proposals
[std_change_proposal]
Variables on User Groups
[sys_user_group]
Question Choice
[question_choice]		 Choice values & price data for choice question variables.
Question Regular Expressions
[question_regex]		 Regular expression validations that can be used on question variables.

System variables

There are many areas within the system of ServiceNow which utilise arbitrary forms and fields, including Integration Hub and Connections. These are typically stored within the below tables.

Table Notes
Variables
[var_dictionary]		 Holds the definition of the variables.
Value
[sys_variable_value]		 Holds the values of variables. A lot of Workflow (not Flow) and Integration Hub values live in here.
[var__m_*] Auto-generated tables to hold variable values.
E.g. [var__m_sys_hub_step_ext_output_4f1590a253a1301026b0ddeeff7b1279]

Variables relating to the ServiceNow Automated Testing Framework.

Table Notes
Test Variable
[sys_atf_variable]		 Extends "Variables" [var_dictionary]
The parent for all of the different kinds of ATF variables
Input Variables
[atf_input_variable]		 Extends "Test Variable" [sys_atf_variable]
Output Variables
[atf_output_variable]		 Extends "Test Variable" [sys_atf_variable]
Parameter Variable
[sys_atf_parameter_variable]		 Extends "Variables" [var_dictionary]
Table Notes
Flow Data Variable
[sys_flow_data_var]		 extends "Variables" [var_dictionary]
Flow Catalog Variable
[sys_flow_cat_var]		 extends "Variables" [var_dictionary]
Table Notes
Activity Variables
[wf_activity_variable]		 extends "Variables" [var_dictionary]
Workflow SC Variable
[wf_variable]		 extends "Variable" [item_option_new]