Using definite article before abbreviations

Consider this part of a technical article.

Approaches to boilerplate detection typically exploit DOM- level features of segments by means of handcrafted rules or trained classifiers, or they identify common, i.e., frequently used segments or patterns/shingles on a website ..... Yi et al. simplify the DOM structure by deriving a so-called Site Style Tree which is then used for classification [26]. Baluja [2] employs decision tree learning and entropy reduction for template detection at DOM level.

DOM is abbreviation for Document Object Model, in this article sometimes it is used with "the" and sometimes without it, Why? As I checked another document, they may use the DOM tree but DOM trees.

I guess as the DOM tree and the DOM structure is unique, they use "the"

In general, are there rules for definite article and abbreviations?