PLOT-10 is a set of vector-graphic commands for 1970s-era Tektronix 4010-series graphic terminals, which were mostly used as streamed data sent to such a terminal, but sometimes stored as a file format as well. They were usually used in conjunction with FORTRAN programs. These terminals differed from the more common raster-based video displays (which use bit-maps where each pixel of the screen corresponds to a group of bits in memory) in that there were no directly memory-mapped pixels, but rather, the terminal took commands to draw points, lines, and shapes in particular positions and directions, which then physically persisted on the phosphor screen without needing to be refreshed from memory; the only way to remove a shape was to clear the whole screen, making this sort of monitor unsuitable for animation.
Details on the format associated with such long-obsolete devices could be difficult to come by; as FileFormat.info puts it, "If you need this information, you really need it badly, and good luck." However, the efforts of historical archives such as the Internet Archive have made some of the original documentation accessible now.