(Short version: Yes it does mean it could hold X litres of water, but that’s not why they give you the information.)
In the metric system a litre is a unit of volume.
Volume is essentially another word for “size” in three dimensions – height, width and depth – all multiplied together.
It might seem like a litre is a unit for “amount of liquid” but that’s not actually the case – it’s just that we choose to measure amounts of liquid by *volume* rather than by *mass* (in grams or kilograms).
The volume of the oven is very important to know when considering how much food you’ll be able to cook inside it; as well as effecting how efficient it is. A bigger volume will be harder to heat up (taking more energy) but will allow you to cook more food simultaneously.
But it’s even more valuable to know the individual dimensions of the oven – is it the right width for your baking trays? Is it deep enough for them?
A 50 liter oven can contain 50 liter of stuff, like 50L of any liquid.
In the metric system, conversion between liters and cubic decimeters (or any order of magnitude) is very convenient. 50L equates 50 cubic decimeters (1dm = 10cm), which can for instance correspond to an oven with the dimension 5dm x 10 dm x 1dm, or any other sum that equates 50.
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