Card types
twitter:card chooses the layout. summary is the compact card, with a smaller image beside the text. summary_large_image places a wide image above the title. If the tag is missing, this tool estimates the summary layout, which is the historical default, and says so under the card.
Player and app cards need more than an image and a title. Those values are listed in the metadata table, and the visual estimate stays on the summary layout rather than pretending to render a video player.
Fallbacks
X can use Open Graph tags when the Twitter-specific ones are absent. twitter:title falls back to og:title, twitter:description to og:description, and twitter:image to og:image. The checklist still marks a Twitter image as missing when only the Open Graph image exists, and the card notes the fallback it used.
twitter:site and twitter:creator are the site and author handles. They do not replace the title or image. For the image file itself, including size and load errors, use the OG image checker.