Use only the sub categories from a category input I have a category input. The category group has two levels. 



When accessing the category input via Twig I am trying to only grab sub categories the user selected.  However when I use 

{% set firstCat = entry.categoryInput.first %}
{% set cats = firstCat.getDescendants() %}


or children it grabs all the descendants from Categories > Group not just from the structure that was selected from the input.
You could set the level parameter according to your requirements:

{% set cats = craft.categories({
    relatedTo: {
        sourceElement: entry,
        field: 'categoryInput'
    },
    level: 2,
    limit: null
}) %}