What it means is, the calendar length of the flowering cycle will vary depending on how many hours light/dark you use during flowering.
Contrary to internet "custom" its perfectly possible to flower plants under lighting cycles OTHER than 12 hours light/12 hours dark and there are several reasons why you might consider using other than 12-12 cycles.
For example. flowering under 11/13 (or even 10/14) will cause plants to finish faster (in some cases by 1-2 full weeks), and is especially helpful with sativa-dominant plants that never seem to want to finish up.
Alternatively, you can run your lights 13/11 or even 14/10 in flowering to increase final yield, at the cost of a longer flowering time.
Some people like to start flowering at 14/10 then reduce the amount of light by 30 minutes each week, to end at 10/14. This more closely simulates what happens when plants are grown outside and winter approaches, and some thing it improves yield and quality.