Using category as the control field for custom field values

[zelena]

I have a custom field that I would like to use as a tags field - with different tag values available to select depending on the category of the listing. For example:
Cat 1 -> tag1, tag2, tag3 available to select
Cat 2 -> tag3 only available to select

I have the impression this is possible as the following text appears when setting up an array of values:
"You can use the {fieldvalue}, {category}, {parent_category}, {listing_type} tags. The category tags will work only if you modify the click2search urls for each field to use the 'cat' parameter instead of 'criteria'. The listing type tag will only work if either 'cat' or 'criteria' parameters are used in the URL."

However, I think I am misreading or misunderstanding, because I cannot get this to work at all. 

Any pointers for using category as the control field? 

Thanks in advance.

[Alejandro]

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