There are three British Standards for straightedges, covering toolmakers' straightedges, the large bow section (camel back) and I-section straightedges, and straightedges of rectangular section steel or granite. Of those three, only the rectangular section steel are likely to interest most woodworkers.
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Straightedges are used as references in engineering, and using them to draw lines (especially with a knife) is bad practice because it abrades the reference edge. A straightedge with slivers knifed off the edge is no longer straight. Straightedges are precision tools, and thus expensive to make (especially the longer cast-iron camel-back ones).
Straightedges do not have scales - they are straightness references, not rulers.
Woodworkers have for generations made their own wooden straightedges; the principle of making three and using them to compare against each other has been mentioned a couple of times in this thread. Clearly, using these with a knife is likely to degrade them quite quickly.
Woodworkers have traditionally used all sorts of things to mark straight lines or test straightness of work - long spirit levels, eyed-up pieces of stock wood, bits of aluminium angle and so on. Used with common sense as to the accuracy needed, such improvised tools usually work perfectly adequately. When something more accurate is needed for setting up and testing machines, plane soles and so on, investment in the proper tool at least gives peace of mind about HOW accurate the straightedge is.