Symbols thrown in by text editors

Sometimes when a file is opened with some editor (in my case a cfm开发者_运维百科 file with ColdFusion Builder), it displayes the text littered with symbols such as [] or �. Can anyone tell me what the proper term for those is (so I don't sound like a troglodyte when referring to them as black rhombus question mark, unless that is the proper term in which case I am sorry), and why they are generated (I'm guessing markup annotations that can't be parsed out)?

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�, code point U+FFFD, is named REPLACEMENT CHARACTER in Unicode. In normal contexts, referring to it as "the Unicode replacement character" would be the normal thing to do. This character is used when the data being read does not correspond to any valid character.

A simple box is used by some systems to indicate a character for which no glyph is available (i.e. no available font provides that character); other systems use a box with hex digits inside representing the Unicode code point, or other glyphs. I don't know if it has an official name, but "fallback representation" would be appropriately descriptive.

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I've always called them invalid characters.