Halliday & Matthiessen (2004: 582, 583):
Unlike reference and ellipsis, the resource of lexical cohesion is, in general, neutral with regard to textual statuses; … A given lexical item can be assigned any of the textual statuses; it is not predisposed to any particular status. Similarly, a given lexical relation is also neutral with respect to textual statuses. However, a particular lexical chain in a text is very likely to show some systematic pattern in relation to textual status. … Texts also vary in how lexical chains relate to textual status, but this variation usually reflects systematic strategies associated with the registers that the texts belong to. … Lexical chains may thus be foregrounded either as Theme or as New.