“Special character” has no fixed meaning, which is why this job goes wrong so often. To a database import it means anything outside the characters it expects. To a file system it means slashes, colons and question marks. To a person pasting from a Word document it means the curly quotes and long dashes that turned up uninvited. This tool lets you choose how strict to be and then does the removing.
The three levels
“Letters, numbers and spaces” is the usual choice. Every letter from any alphabet stays, so names with accents or non-Latin scripts survive, along with digits and whitespace. Punctuation, symbols and emoji are removed. Note that apostrophes count as punctuation here, so “don’t” becomes “dont” and “O’Brien” becomes “OBrien”.
“Letters and spaces” goes one step further and drops the digits too.
“Plain ASCII” is the blunt one. It keeps only characters you could type on a basic English keyboard and deletes the rest, accented letters included, so “café” becomes “caf”. If you would rather turn “café” into “cafe”, run the text through remove accents first, then this.
Line breaks are kept by default. Untick that and the lines join up with single spaces, which is useful if you are producing one string for a field that cannot hold a line break.
Where this is useful
Preparing names for a system that rejects anything unusual. Building a clean string for a file name; for URLs the better tool is the slug generator, which also swaps spaces for hyphens and handles accents. Cleaning up text pasted from a web page that carried odd bullets, arrows and box-drawing characters along with it. Getting rid of the “smart” characters a word processor inserts, though for quotes and dashes alone the smart quotes converter changes them into plain versions instead of deleting them, which is usually kinder to the sentence.
What it will not do
It deletes; it does not replace. “R&D” becomes “RD”, not “R and D”, and a price loses its currency sign. If you need substitutions, find and replace with a regular expression gives you control over what each character becomes.
Zero-width characters are another thing to know about. They are invisible and they count as symbols or format characters, so the first two settings remove them as a side effect. That is why running text through this tool sometimes fixes a problem nobody could see.