Can we apply the +/- 15 V continuously for ABL6TS02B? & would the voltage on the secondary be adapted?
Yes you can use the ABL6TS02B without impact on long run.
The Phaseo ABL 6TS single-phase transformers offer is designed to supply control circuits in electrical equipment from a 230 V a or 400 V a supply (depending on the model) at 50 or 60 Hz.
± 15 V connectors at the primary ensure adaptation to the actual values of the supply networks to which they are connected.
This means you can adapt to the primary voltage (from x+15V to x-15V) so that you have the 24V on secondary side
(See picture below)