Use a semicolon to separate two complete sentences not joined by a conjunction. It indicates a closer relationship than a period would.
The company proposes to do X; this will cause Y.
Use semicolons with adverbs like however, therefore, and indeed when joining two complete sentences.
They took immediate steps to address the spill ; however, staff were dissatisfied.
But do not use the semicolon if both clauses being joined are not complete.
A truly efficient method , therefore, was needed.
Semicolons should be used instead of commas to separate list items if and only if one or more list items themselves contain commas.
Supplemental information requests were sent June 25, 2018; November 13, 2018; and January 14, 2019.
The company committed to doing X, which will accomplish A; Y, which will accomplish B; and Z, which will accomplish C.