Many of the symbols are drawn from existing character sets or ISO/IEC or other national and international standards.
Unicode focuses on symbols that make sense in a one-dimensional plain-text context.
For example, the typical two-dimensional arrangement of electronic diagram symbols justifies their exclusion.
For adequate treatment in plain text, symbols must also be displayable in a monochromatic setting.
Even with these limitations – monochromatic, one-dimensional and standards-based – the domain of potential Unicode symbols is extensive.