There is no maximum resolution, nor any specific scaling rules, that apply for TrueType fonts. TrueType fonts are based on a relative, fundamental coordinate system (called an em square), with mathematical equations to "grid-fit" the characters in that coordinate system. To scale and/or print the font, a transformation is made from relative to absolute values based on the output device's resolution in pixels.
Likewise, PostScript fonts do not have a DPI limitation. They are also based on coordinate systems and mathematical equations (like TrueType) to translate between relative and absolute values, in order to "map" to the resolution of the output device.
In terms of comparing the technologies, PostScript is a full-fledged page description language, whereas TrueType only deals with font technology - fonts are only a subset of more major technologies. It would be more appropriate to compare PostScript to QuickDraw GX as they are both the major technologies.
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