It is very important that you know which method your LED fittings use before any cables are run, so the electrician can install them correctly.
This is, however, only relevant to light fittings which are low voltage and have a separate driver/transformer, as mains voltage fittings are always wired in parallel. A manufacturer or supplier should include this in any technical details for their light fittings.
So, what are the differences?
Parallel wiring is used for constant voltage LEDs (e.g. 12V, 24V), and it is wired the same way that conventional lights are wired within the home.
Series wiring is used for constant current LEDs (e.g. 350mA, 700mA, 1050mA, etc). They are connected differently to allow the current, rather than the voltage, to be kept at the same value. This ensures the maximum output of light and stability is achieved.
It is important to understand the risks of not wiring LEDs correctly. It could result in the LEDs not working, or potentially even being damaged or destroyed. If you need to rewire at a late stage of a project, it could result in high additional costs and repair work since the cables might need to be changed. This demonstrates that it is much better to plan early on and avoid these potentially costly, and time-consuming, changes.