What gets encoded
Text mode stores the characters you type, including spaces and line breaks. URL mode accepts only a full http or https address, and it stores that address as written after trimming the ends. It does not add a tracking parameter, and it does not send the address to a shortener. The image is a static code: a scanner reads the payload from the picture. Nothing on a server can expire it or change where it points, because nothing is stored.
An empty box does not produce a code. A value such as example.com is rejected in URL mode until it has https:// in front; switch to Text if you want those words exactly. Very long input is rejected when it no longer fits at the correction level you chose. The message says so, instead of drawing a code that cannot be scanned.
Size, correction, and margin
The default error correction is M, which can recover about 15 percent of damaged modules. L holds more data and recovers about 7 percent. Q and H recover about 25 and 30 percent and hold less. The quiet zone is four modules of background on every side, the margin the QR specification asks for. The PNG uses a whole number of pixels per module so the squares stay sharp, and the image is about 1024 pixels on a side. The SVG is vector artwork.
Color and contrast
The default is black on white. If you change the colors, the foreground has to be darker than the background, and the contrast has to be at least 4.5 to 1. Low-contrast and reversed colors are rejected instead of being offered as a download. Stillmark does not place a logo on top of the code.
Privacy
The code is drawn in your browser. The text or URL is not uploaded, not logged, and not wrapped in a link that this site controls. There is no account and no analytics script. PNG and SVG downloads are files your browser saves. Fonts are included with the page. Reloading the tab clears the box.