When any fundamentally important document is drafted which may be a grundnorm (Fundamental norm to support all other legal norms) of that system, then generally it is differentiated from the ordinary municipal laws by referring to its clauses as articles rather than sections, such as United Nations Charter, International Conventions, Constitution of a country etc. from where other laws or rules originate.
Not necessarily that a Constitution has always articles but no sections. Article is used to convey an impression that they are more elaborate and conventional in expression in olden times.In modern times most of the Act prefer to use 'Section',which is more simple and direct.
Illustration: Think of the ways a book is often broken down or divided into smaller portions. A single book may have a handful or more of chapters. Each chapter may have tens or hundreds of paragraphs. Each paragraph may have tens of lines. Each line may have several words. Each word has anywhere from one to tens of letters. One of the easiest books to use to illustrate this would be the bible; it can be divided by testament, book, chapter and verse.
Even more on point, think of the way an outline looks. At the top of the page and the furthest to the left is roman numeral I. With my outlines, I then use a capitalized A placed below and to the right of the Roman numeral I. Then, below and to the right of the A comes an Arabic 1. Then below and to the right comes a lower case a. Finally, lower case Roman numerals are used.
I
A
1.
a
i.
With regard to the Constitution of the United States of America, it can be broken down by article, then section, then paragraph, then clause. For example, if you wish to find the section that empowers the House of Representatives the sole power of impeachment, you would go to Article I, section 2, paragraph 5, clause 2.
