Custom Component Tiled navigation

What's deciding where to pull the nodes from is this bit of code

<#if coreNode.nodeType?lower_case == "community">
    <#assign whereClause = " WHERE depth=1 " />
<#else>
    <#assign whereClause = " WHERE parent.id = 'category:${coreNode.id}' " />
</#if>

so if you want to be able to choose a specific category, that's the bit of code to modify.

The most versatile way to modify this is to add a bit of code to support parameters, so you can pass a category ID to it, and it will pull from that.

To get a parameter passed to a component you use

env.context.component.getParameter("parameter_name")

So the top of your code might look like 

<#assign useCategory = env.context.component.getParameter("category")!"" />

...and then we need to modify the if/else clause I highlighted above to use this. We're fetching a parameter called "category", and assigning it to useCategory, and have a default of empty string, in case the parameter isn't present.

<#if useCategory?has_content>
    <#assign whereClause = " WHERE parent.id = 'category:${useCategory}'" />
<#elseif coreNode.nodeType?lower_case == "community">
    <#assign whereClause = " WHERE depth=1 " />
<#else>
    <#assign whereClause = " WHERE parent.id = 'category:${coreNode.id}' " />
</#if>

So here we're checking if useCategory exists and is not an empty string, and if so, we use that, otherwise we'll fall in to the logic that existed before.

To use this component with a parameter, it would look like:
<@component id="component.name" category="TheCategoryIDHere" /> (freemarker)
or
<component id="component.name" category="TheCategoryIDHere" > (Quilt/Page XML)