Difference between positional isomerism and metamerism (organic chemistry)

I don't want the definition of positional isomerism and metamerism but want what differentiates the two types of isomerism.

I know metamers have different alkyl groups on either side of it, but doesn't that just mean the position of the functional is changing? Hence shouldn't it be a positional isomer.

If functional group carbon is part of principal chain then metamers are also positional isomers. In case of polyvalent functional group like sulphoxide($\ce{>S=O}$), secondary amine ($\ce{-NH-}$) etc. structure formed by change in structure of alkyl groups will be metamers only. For example, diethyl amine ($\ce{Et2NH}$) and methyl propyl amine ($\ce{MeNHPr}$) are metamers.