The key purpose of the "I before E except after C" rhyme is to enable those learning spelling to spell words correctly. "I before E except after C, unless when sounding like "ay" as in neighbor and weigh" refers to the E and I sequence in words.
Each time there are vowels like E and I just after a C, the instruction is to write E first before writing I. For instance, deceitfully and deceivingly. When the sound of the word is like "ay," the rule is to write E first before writing I. For instance, veil, sleigh and eight.
In other cases where there are E and I, write I first before writing E. For instance, die, thief and priest. There are exceptions to these rules. For instance, foreign, caffeine, ancient and sufficient.