Bar mounts - Most manufacturer's have a specific bar mount pattern for the saws they manufacture. Over the last decade many of the manufacturer's have standardized, but there are still many distinct bar mounts. While the bar may fit the saw, the chain adjuster and oiler holes probably will not line up correctly unless you use the application specific bar.
Saw Chain pitch - There are 3 common pitches of chain with many different cutter configuration. Standard chain pitchs are 3/8, .325 and .404. Normally consumer chainsaws use a variation of either 3/8 or .325 pitch. 3/8 pitch is available either as 3/8 lo pro (low profile) used on smaller consumer saws up to approximately 2.7 cubic inches and bar lengths up to 18". Regular 3/8 chain is used on sas 2.7 cubic inches or larger on mostly farm or commercial chain saws. Then there is the .325 pitch (what we in the south nicknamed "metric chain" in the early 1990's) used on many imported saws like Stihl, Jonsered, Husqvarna, Sachs Dolmar, etc. The .325 pitch chains have gained widespread acceptance and are now used on mid-size saws regardless of where the saw was manufactured.
Saw Chain Gauge - Common Sawchain gauges are .043, .050, .058 and .063. This gauge refers to the width of the drag link on the bottom of the chain. The drag link is the part of the chain that rests inside the groove of the bar. These drags are not interchangeable. While you can fit a smaller drag into a larger bar groove this will likely cause the chain to cut off angle.
So it is easy to see that when you are shopping for a replacement bar or chain that more information is needed than just the name on the saw and length of the bar.