What are supplementary Unicode characters?

Supplementary Unicode characters refer to those characters that fall outside the Basic Multilingual Plane (BMP). The BMP is a specific range of Unicode characters that includes the first 65,536 code points (from U+0000 to U+FFFF), which encompasses most of the commonly used characters from various scripts, including many modern languages.

In contrast, supplementary characters occupy higher code points, from U+10000 to U+10FFFF. These additional code points are necessary for representing a wider variety of characters, including historical scripts, certain mathematical symbols, and emojis, among others. Since these characters cannot be represented using a single 16-bit code unit (which is what BMP characters utilize), they require Unicode's surrogate pair mechanism for encoding in UTF-16 format. This complexity highlights the significance of these supplementary characters in expanding the range of representable symbols in text.

The other options focus on aspects that do not accurately define supplementary Unicode characters. The BMP includes many foundational characters but does not cover the full scope needed to represent all global languages, while defining characters based on a size limit of 64K does not capture the broader aspect of character categorization in Unicode.