1. Asterisk key. Its original use was to remove an incorrectly stroked outline. In the age of CAT, it has a lot of other responsibilities, differentiation being only one of its duties. BTW, the -Z key is also used for differentiation as well.
It can be used to denote prefixes, capitalization, suffixes, letter-by-letter spelling, and word differentiation. When you are starting out, it is used most of all for correction by itself.
2. Most right-handed people use the right index finger for the asterisk key by itself, as it is the dominant hand. Left-handed people use the left or the right index finger, depending on preference or what keys were used on the previous stroke. I am left-handed, and I don't use my left finger exclusively. I have a double-wide asterisk key on my professional steno machine. It makes it easy to use either finger. Which finger I use depends on what I am stroking. I have freakishly short index fingers, so I don't want to have to twist my hands into odd positions to add the asterisk key. If you are using a regular steno machine and have tiny hands, it might make sense for you to get a single-wide (extended to the right) asterisk key if you are right-handed. If you are left-handed, you might want to get the double-wide asterisk key (extended on both sides). There is a gap between the asterisk key and the surrounding keys. Some people, however, like to keep the gap, as it prevents accidental hitting of the asterisk key. Some folks dislike their fingers getting caught in the gap and prefer the extended key. Changing a keytop on a professional machine isn't a big deal. It is all a matter of preference.
In the above example, you can press it with either. You're doing more on the left side than the right. If you're right-handed, you'll probably tend to use the right. On that particular outline, I'd probably use the right hand, even though I'm left-handed. As Robert Fontaine whose response is below mentions, current steno theories are immensely slant toward the right-handed user. I have had to modify my writing significantly to move some of the steno "load" to the left hand side and make as many strokes as possible "balanced," meaning an almost equal number of keys being used on either side, whenever possible.
Best regards,
Priscilla Trillo