Partial Mortgage Repayment: capital vs interest?

[nomorejob]

In the context of repaying part of the outstanding value of a tracker/variable mortgage, i heard few people saying "make sure to repay the capital and not the interest". I don't understand what they mean as IMO any early repayment can only come off the principal.
Or is there confusion with early partial repayment of a fixed mortgage?

 

[LDFerguson]

If making a lump sum repayment off a mortgage, you have to make it clear to some lenders that you want it used as a repayment off the capital. Otherwise it can be held on account.