I have read numerous posts about making variables visible and even required on Catalog Tasks, but not on Requested Items.
Based on various answers and potential solutions I have designed two options that we use
Option 1 - create variable on RITM level
First step is to create a variable on RITM level and define its visibility using UI Policy
Next, a client script (onSubmit) has been applied on a catalog task. One of the conditions is catalog task short description (not best practice, probably task sys_id could be used). By using short description field in script conditions, we can apply this rule to any task regardless on its order in a workflow; i.e., Task 2 instead on Task 1.
Finally, in the workflow we added this variable to selected variables on catalog task where we want to make this variable visible.
Option 2 - create variable on Catalog task level
This approach creates variable on the workflow and it is visible on the RITM form if there is no defined any UI Policy.
First step is to go to workflow/edit workflow variables and add a new variable. Create new variable and add it to catalog task, or preferred catalog task if there are more than one.
In this approach onSubmit script is created on catalog task level (can be created from any catalog task).
