Halliday & Matthiessen (2014: 473-4):
Here one clause is presented as an alternative to another. Examples of clauses linked by the alternative relation:
||| Either you go ahead || and take the plunge || or you wait || till you think || you can afford it, || which you never will. |||
||| Can I go on the computer, || or have something to eat. |||
|| Guided tours of the Cathedral take place the first Sunday of every month, || or a self-guide booklet about the Cathedral can be picked up inside. |||
||| Did you have to educate yourself about traditional culture and mythology || or did you grow up with that? ||
||| The melt is then cooled at a few degrees per hour || until crystals start to form, || or alternatively the flux is evaporated at a constant rate. |||
Here one clause is offered as alternative to another. The correlative pairing is either – or, and the associated cohesive conjunctions include conversely, alternatively, on the other hand.