The stages run top to bottom: trim → drop blank lines → remove duplicates → sort → add numbers. The same settings always produce the same list.
The default is dictionary order, which puts "item2" before "item10". Ticking this sorts by character code instead, which groups symbols and capitals first.
Paste some lines and the result appears here.
Paste a list and strip the duplicates and blank lines, sort it, and number it. Pick only the stages you need, and see how many lines survived and how many were dropped. It runs in the page, so a list of addresses or customer names never leaves your machine.
Questions
Is there a size limit?
None by design. Tens of thousands of lines are fine, though typing may feel less immediate on a modest device.
Is my list stored?
No. Everything happens in the page and nothing is sent to a server. Close the tab and it is gone.
Why does Japanese not sort into kana order?
Because the reading of a kanji is not recoverable from the characters — 東 may be read higashi or azuma depending on the word, and no tool can decide that from the string alone. A spreadsheet can do it only because it remembers the reading you typed, and that memory is gone the moment the text is copied out as plain characters. To get kana order, keep a separate reading column and sort that. Lines written purely in kana do sort roughly into kana order.
Do the line numbers end up in the copied text?
Yes. Copying takes the numbers with it, so untick the option if you want a plain list. If you send a numbered result back to the input, the numbers become part of the line text on the next pass and will be matched and sorted along with everything else, so stacking passes on a numbered list is rarely what you want. The format can be a full stop, a colon or a tab; the tab form splits into two columns when pasted back into a spreadsheet.
Can I use it on a spreadsheet column?
Copy the column, paste it in, and it comes back in the same shape ready to paste back. If you paste several columns at once the separators arrive as tabs and the whole row is compared as one line — to deduplicate on a single column, paste only that column. A cell containing its own line break arrives as several rows and shifts everything under it, so fix those cells before copying.
For Japanese input on Windows: Asteria