ellipsis -So

In general the term ellipsis refers to any omission of a word or words.

Linguistic ellipsis occurs when material is omitted from a construction but can be recovered by looking at the previous linguistic context.

So replaces missing elements of the predicate but it functions in two distinct ways.

If often behaves very much like too, replacing everything after the operator, although so also forces subject/operator inversion in the second predicate---Iris can program computers and so can Fanny. 

So can also replace a clausal direct object construction. In I hate Pam's boyfriend and I told her so, so stands for the entire proposition "I hate Pam's boyfriend," i.e., I hate Pam's boyfriend and I told her [that I hate her boyfriend].