The flag of the United States shall be thirteen horizontal stripes, alternate red and white; and the union of the flag shall be forty-eight stars, white in a blue field.
The "union" of a flag is defined as the "upper inner corner" of the flag. So it does not define the size, but does define where it should be.
Following the practice of British ensigns, a canton sometimes contains a symbol of national unity, such as the blue field and white stars of the flag of the United States of America. In these cases, the canton may be called simply the union.
The flag code does not. There was an executive order placed with how to lay it out but to the letter of the actual law (as best I can tell) this works
From chapter 1 of the flag code:
The "union" of a flag is defined as the "upper inner corner" of the flag. So it does not define the size, but does define where it should be.
The upper inner corner is the canton of a flag, wtf is a union?
maniacal laughing as Canton flips over to reveal a red field with hammer and sickle.