What are the conditions for using skeletal replacement ('a') nomenclature? The Blue Book 2013 in P-63.2.4.1 gives “1-methoxy-2-(2-methoxyethoxy)ethane” as the IUPAC preferred name for $\ce{CH3OCH2CH2OCH2CH2OCH3}$ and comments,


  Skeletal replacement (‘a’) nomenclature [method (4)] generates preferred IUPAC names, when the conditions for using this type of nomenclature are met (see P-15.4); otherwise substitutive nomenclature must be used.


It then names $\ce{CH3OCH2CH2OCH2CH2OCH2CH2OCH3}$ as “2,5,8,11-tetraoxadodecane.”

My question is why skeletal replacement nomenclature was used in the latter case but not the former—why the conditions for using skeletal replacement nomenclature are not met by the former molecule.

P-15.4 states that


  “Skeletal replacement (‘a’) nomenclature is used in two ways:
  
  “ (a) …
  
  “ (b) to generate simpler names for heteroacyclic structures than those formed by following the principles of substitutive nomenclature, for example, in naming polyamines, polyethers, etc., by replacing carbon atoms in corresponding acyclic hydrocarbons” (89).


The former molecule seems like an instance of a polyether, and the name “2,5,8-trioxononane” seems simpler than the given preferred name, so it seems that this molecule meets the this criterion.

What factor led to the use of substitutive nomenclature where replacement nomenclature seemed to apply?
Rule P-51.4, not P-15.4, specifies the conditions under which skeletal replacement nomenclature generates preferred IUPAC names.


  P-51.4.1.1 Skeletal replacement (‘a’) nomenclature rather than substitutive or multiplicative names must be used to generate preferred IUPAC names for acyclic structures when four or more heterounits are present in an unbranched chain containing at least one carbon atom and when none of the heteroatoms constitute all or part of the principal characteristic group of the compound.


The former compound has only three oxygen heteroatoms, not enough to require the use of replacement nomenclature.