What the file needs.
Every print process does something different to the file you upload. Below, for each one: what the machine makers, the RIP vendors and the platforms say they need, where they disagree, and what fails on the floor when a file ignores it. Every value links to the page that states it.
DTG printing
DTG in RGB or CMYK? 150, 300 or 360 DPI, sRGB, not CMYK, black knocked out or underbased on a black shirt: what Printful, Printify, Gelato and Epson ask.
DTF
DTF in RGB or CMYK? 300 DPI at print size, a true transparent PNG, strokes and gaps of 2 pt: what DTF makers, RIPs and shops ask, and where they split.
Sublimation
Mirror sublimation images? On cotton? Mug template size: four platforms, four aspect ratios for an 11 oz-class mug; placement drifts 0.5 to 2 inches.
Wall decor
Canvas and poster print DPI (150 or 300), poster size in pixels, sRGB, 4 mm of bleed and up to 1.5 in lost per canvas edge: what platforms and labs ask.
Screen printing
Screen printing moire, white underbase and halftones: 300 DPI raster, 1 pt strokes, 2 pt gaps, 46 to 55 LPI: what printers and RIPs ask of a file.
Embroidery
Embroidery minimum text size 0.25 in (6.35 mm), lines at least 0.05 in, 6 thread colours from 15: what platforms ask of a logo file, and where they split.
UV printing
UV printing white ink and varnish: spot swatches above the colour, 300 dpi at final size, UV-DTF line minimums of 0.3 to 0.6 mm: what makers and RIPs ask.
HTV and vinyl
Mirror HTV or not? 5000 paths on Cricut, 0.001 mm cut lines on Roland, Poli-Flex's 1 mm minimum size: what makers and cutter software ask of a file.
Resolution and DPI
150 vs 300 vs 600 DPI? 150 for DTG and 300 for DTFlex at Printful, no DPI at Redbubble or on a cut path: floor, recommendation or printer output.
Colour
RGB or CMYK for printing? sRGB or RGB on most platforms, CMYK first at two DTF shops, RGB inside two screen RIPs: what sources state, and who converts.
KDP spine width, bleed and cover size
KDP spine width, bleed and cover size: 300 ppi at KDP, Lulu and IngramSpark, 150 at Gelato, bleed of 0.125 in or 4 mm, a spine set by page count.
Stickers
Sticker cut line, bleed size and white ink on clear: 300 PPI, 1/16 to 1/8 in of bleed, a cut line named by spot colour or drawn as a magenta stroke.
Mug and tumbler template sizes, DPI and seam
Mug and tumbler template size changes per platform and product (2538x945 px, 2718 × 1112 px, 200x96mm): 300 dpi at print size, a seam under the handle.
All-over print
All-over print template per panel, at least 150 dpi at print size, 9038 × 8148 px canvases, print shift from 0.5 inch (Printify) to 2 inches (Printful).
PDF/X-1a vs X-4 and printer PDF rules
PDF/X-1a vs X-4: X-1a at KDP, X-1a or X-3 at IngramSpark, X-4 at Gelato and Cloudprinter, no flavour at Lulu: what each platform asks of a PDF.