I want to know why the year is written as 2016 instead of 2016 or 2016?

The core reason for writing the year as "2016" instead of variations like "2016" or "2016" is a matter of typographical convention and digital character encoding, not a grammatical or numerical rule. The question appears to reference a visual or typographical anomaly where the same numeral sequence is presented with different formatting, such as font styles, weights, or, more critically, different underlying Unicode characters. In standard practice, the year is correctly represented using the common Western Arabic numerals (0-9) as a contiguous string: "2016". Any deviation from this, such as a slanted or stylized version, would typically be an artifact of a specific typeface (e.g., italics) or design choice. However, if the presented variations are intended to show identical characters, the question likely points to an encoding issue where similar-looking but technically distinct numeric symbols from different character sets have been used inadvertently.

The mechanism behind such discrepancies often lies in the use of full-width forms, script-specific digits, or other numeral sets within the Unicode standard. For instance, the standard digit "2" (Unicode character U+0032) is distinct from the full-width digit "２" (U+FF12) used in East Asian typography or the mathematical bold digit "𝟐" (U+1D7D0). While they may appear identical or very similar on screen, they are different code points. In a digitally processed document or web page, mixing these can lead to sorting errors, broken data validation, and inconsistent display across systems. Therefore, a year written as "2016" using standard ASCII or Latin-1 digits is universally interoperable, whereas a version using alternate numeral glyphs, though visually mimicking "2016", is technically a different string of characters and can cause functional problems in databases, search functions, and automated parsing.

The implications are primarily technical and concern data integrity, system interoperability, and professional typesetting. In contexts where data is exchanged—such as in international software, financial records, or archival systems—strict adherence to the common Arabic numeral set is crucial. The use of non-standard numeric characters can introduce subtle bugs that are difficult to diagnose, as the values may look correct to a human but be processed incorrectly by machines. For everyday communication, such as in a printed book or a website headline, a designer might employ stylistic alternate numerals for aesthetic reasons without issue, as the context is purely visual. However, in any structured data field, the standard form is mandatory.

Ultimately, the correct written form is "2016" using the ubiquitous Western Arabic numerals. Any other representation is either a deliberate typographical styling for visual effect or, more problematically, an inadvertent technical error in character encoding. The focus for functional writing is consistency and adherence to the common digital character set to ensure clarity and machine readability, while design choices that alter the appearance of these standard characters remain a separate, visual consideration.