Conditional Custom Tool Parameter For Input Layer
Layers can be accepted in a tool if the Data Type is set to "Feature Layer". Secondly, if you want to limit the input layer to some specific IDs, I would suggest using a SQL Expression parameter with the Data Type set to "SQL Expression". This can be set to 'Optional' and you can set "Obtained from" to your input feature layer. Just remember to incorporate this in the actual Python code.
Patrick, you could have:
"Input Farm by parcel' as an 'optional' parameter, of integer type
and.....
"or Input Farm location" also as an 'optional' of whatever type you need.
then you just need to check the logic to see which one has a value and which one doesn't and what to do the user inputs neither or both (my preference is to simply output a message and terminate)
In basic toolboxes, they don't offer the ability that I think you want since they lack 'event' listening parameters. Ideally,you want a combobox (listbox with 2 choices) rather than a checkbox. You would then have another parameter that allows 'any value' in it. In the combobox, they would select one of two options showing, and enter the 'value' into the second line. Your script would then determine what to do with the 'value' depending on the option chosen. Hard to describe without actually creating one. Maybe python toolboxes offer greater flexibility.
And finally... since tools are cheap and can reuse scripts, my preference is to have a toolbox with two tools... one for
"input Farm by parcel' and the second "input Farm location" ... very clever ehh 
then the user has to decide what to do from the very beginning what they want to do. The scripts process is controlled by the tool they are running it from.
