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Note: If you want to work with only one version of jQuery you must delete jquery-1.4.2.min.js file,  enable jquery on edit-item-form.jsp layout tag and delete jquery-1.4.2 library reference.

The advantage of using variables is that our pages will load only necessary script libraries and it makes easier to update library versions site-wid. And you can change jQuery for any other script library of you choice or add more libraries.