of those that are in heaven, on earth,
Not to be a downer, but IHS is actually a traditional abbreviation for IHSOUS, transliterated from the Greek spelling for “Jesus.” So, it is actually a Latinized abbreviation of the holy name of Jesus, despite later attempt, such as this one, to give it meaning as an acronym. St. Bernard of Siena is famous for having preached a devotion to the holy name of Jesus and the “IHS” symbol was the stylized version of the holy name that he used. As evidence of this, this symbol is still found through the city of Siena and beyond, as a symbol of devotion to the sacred name of our Lord.
For what it’s worth, another popular (but erroneous) explanation is that IHS stands for In Hoc Signo, from the well-known Latin phrase “In hoc signo vinces” or “in this sign, conquer” in reference to the cross.
Just my two-cents. :-)