Union Civil War era orders came in two varieties: general and special.
General Orders affected large numbers of people in a department, district, or large command. One notable example is General Orders Number 11 for the Missouri District of the Border which ordered three and one half Missouri counties all but depopulated. This was the 11th General Order issued for that district in the year 1863.
The orders were numbered as they were issued and started anew with each calendar year. So, the first order of any year would have been Special Orders Number One and so forth. Each separate command level issued its own orders.
Now, Special Orders concerned fewer numbers of people than did general orders, and sometimes were issued for individuals. Special Orders were issued for a wide variety of reasons, which could be positive or negative to the individuals named in them.
All that is to say that a researcher would need to know more about who issued Special Orders No. 166 and when it was issued in order to help you. Or, they could approach it from another direction of research if you could tell us the complete name of the soldier, his county of residence, his date of birth, and the regimental number of his Enrolled Missouri Militia unit. Can you give us more to help you?