A catalog item defines a service someone can request. Variables collect the information needed to fulfill that service, such as delivery location or business justification. They are inputs to the request process, rather than ordinary incident fields. Use a variable type that fits the input: a reference for a user, a choice for a controlled option, or text for a justification.
Mandatory variables prevent incomplete requests at ordering time. A variable label may resemble a field label without being the same database field. Requested for identifies the person receiving the service, which can differ from the person submitting the order. Read the item requirements before ordering. Empty justifications and ambiguous delivery locations cause fulfillment work even when the item itself was selected correctly.