"Remanent" is an incorrect spelling when referring to a remaining portion; the correct term is "remnant," which means a small remaining quantity of something. How to spell Remnant? Picture a fabric "remnant" as a leftover piece after an outfit is made.

The rest just refers to another part after you've set aside a portion. The remainder is similar. Both are rather general in meaning. The remnant refers to something that is left over after it has been used up or consumed. This is often used to refer to additional material or cloth.