In the rules, you have core units and optional additional ones of various kinds. There are army lists provided for a number of conflicts in the period between 1700-1865. These are; the French and Indian War, the American War of Independence, the Peninsular War, the American Civil War and the Indian Mutiny. Other army lists will appear over time for other conflicts and there will be supplements to cover specific wars.
Anyway, of immediate interest here is the ACW, so here are some photographs of my Union forces. All figures are Peter Pig 15mm. Groups of figures are on movement trays by Warbases. In SP, standard infantry and cavalry consist of groups of eight, skirmishers are in groups of six and irregulars come as groups of ten.
First, here are five groups of eight Union infantry, accompanied by NCOs;
Next, we have two units of Union cavalry, each consisting of eight troopers;
And here are the same cavalry dismounted, showing the horse holders;
The SP rules say that when dismounted each group of eight cavalry becomes six active troops plus two horse holders, but, as Peter Pig figures come in packs of eight I painted them all up. I'll probably use the spare figures when I come to making up an Engineer Group, which is one of the generic support options available in the rules.