A common place to use a sash curtain rod is on a door or on a side light window. Typically, sash rods are either flat or round in shape and come in adjustable sizes from just a few inches up to about three or four feet. If you need to cover an area larger than four feet or so, it's probably a good idea to select a standard curtain rod with a small clearance instead. It'll do a better job in a wider span.
When picking out a sash curtain rod, pay attention to the way they're packaged. Many smaller sizes are packaged as a pair. That's because it's common to use these small rods on doors where you'll need a both a top and bottom rod to keep your curtains in place while the door is being opened or closed.
The minimal clearance of most sash rods limits their use to sheers or unlined fabrics. Take note of the clearance of the rod you're interested in and make sure you'll have enough room for your drapery if it's lined or interlined. Also, use caution with thicker fabrics. Sash rods are not designed to hold very much weight, in this circumstance a lockseam rod would be a better option.
Flat sash rods are more commonly used with the rod pocket at the top of your sheers. And, while you can also use a round rod with a rod pocket, you can add very small rings to your fabric if you want a more decorative look.