Lexis As Most Delicate Grammar

The most delicate features on the lexicogrammatical stratum are not lexical items.  The cline of lexicogrammatical delicacy is not a scale of increasingly delicate lexical items.

Lexical items are the "output" of the network.  Each lexical item is a "bundle" of co-selected features, just as the phoneme /b/ is a bundle of the articulatory features {bilabial, voiced, stop}.

Halliday & Matthiessen (1999: 198-9):

The paradigmatic strategy … is typically associated with feature networks: that is, networks made up of systems of features, such that each lexical item (as the name of a thing) realises a certain combination of these features selected from different systems within the network — a particular clustering of systemic variables. … This resource, the construal of systematically related lexico-semantic sets, illustrates well the principle of “lexis as most delicate grammar”. …

Note that it is not (usually) the lexical items themselves that figure as terms of the systems in the network.  Rather, the systems are systems of features, and the lexical items come in as the synthetic realisation of particular feature combinations.  Thus lexis (vocabulary) is part of a unified lexicogrammar; there is no need to postulate a separate “lexicon” as a pre-existing entity on which the grammar is made to operate.