Spot yield curve and yield to maturity

Hi sir, Wanted to ask, are the above terms same? Normally for a bond we have different spot yield curve for each year but only one yield to maturity (IRR), and I read the Acca technical article BOND VALUATION&BOND YIELD (refer to example 1&3 of the article) in this regard but still don't understand why sometimes we need to use different spot yield curve to get the bond price but sometimes only one IRR is used to discount the interest (paid over the bond periods) and principle for getting the bond price? Thanks for your help sir.