Aoife is an alternate which means she goes on set days a week regardless of whether the main lead is ill/on holiday or not.
A standby is like an understudy but they have no other role in the production. They go on when the part they cover is on holiday or has a planned absence.
Then the understudy is a member of the ensemble (generally) who goes on last minute if the lead becomes ill.
So if the story in the paper is true, Tanya Nyari will go on if the lead has a planned absence on a Tuesday or a Thursday.
You would not employ someone to only be a standby for 2 set days a week that would be crazy,if they have named days to perform it would be as an alternate,wording by the paper dosent mean its certain that she has been hired as only a standby.
Its not an exact sceience each producer uses whatever title they want to and most seem to do what they do with Joseph and Jersey Boys a certain performer will be Narrator/Frankie at certain performances.