What steel?

[Frank S]

Never a stupid question is asked the only stupid questions are those which never get asked.
When most refer to a piece of stock they are simply stating that they are using some type of metal which they probably have lying around depending os the tool or part they are making this actually could be just about anything. sometimes even plastic or wood. But often in their posts you will be able to read little subtle hints as to what metal they are referring to.for machinist jacks those can be made out of just about any material you are comfortable with or a combination of metals like using a common bolt for the jack screw then brass aluminum mild steel cast iron Stainless steel or even keeping it real simple just use an elongated nut commonly called a coupling nut.
for other tools such as say a mount for a tool post grinder again most any type of machinable material may be used many prefer to use aluminum simply because it is easy to machine.
If any tool or fixture is going to require welding then mild steel is often used but not an absolute as there are welding and bonding processes for almost any type of metal some more complex than others. Joining dissimilar metals ca also be accomplished across a wide range of metals but not all Obviously one would not want to try and weld stainless or other steels to aluminum.
when making things like punches dies and cutters you need to use metals which can be annealed if need be,for processing then hardened.
f\for making clamping tools I find that when ever possible I chose metals with high tensile strengths but still machinable and not so hard as to be brittle
Hope this helps I'm sure there will be others who will add more input and welcome to the the best place to feed TAS ( Tool Acquisition Syndrome) may it become an addiction that you will never want to be cured of.