What is $source and $sources in BaseRelationField?
In case of the element type Asset sources are the available volumes. In case of Category the available sources are the different groups. Sources for Entry are your sections, Tag have the different sets/groups as well.

It contains an array or the type section, folder, group and the ID of the corresponding model, separated with an :

You can just do 

$field = Craft::$app-&gt;getFields()-&gt;getFieldById(&gt;&gt;your field id&lt;&lt;);
Craft::dd($field-&gt;sources);


to get to know how it looks like. It could be like this:

array(1) {
  [0]=&gt; string(10) "section:16"
  [1]=&gt; string(10) "section:18"
}


or for an Asset

array(1) {
  [0]=&gt; string(8) "folder:1"
}