Are you referring to
$(document).on('change', '#client', function(){
// ...
});
If so: You are saying to monitor the tree with the root of (document) for any nodes that match the selector '#client' and to bind the event 'change'. It is better to use a more localized subtree instead of root, for example the form. If the form had an id of 'form' then you could do
$('#form').on('change', '#client', function(){
// ...
});
This only needed when the element is dynamically created. Another example: You get a set of data via ajax and build out a table. In that table there are edit and delete links that are generated for each row. You could do something like:
$('#table').on('click', 'a.edit, a.delete', function(){
// ...
});