As you have seen, the data-model is basically a tree. This tree can be arbitrarily complicated and deep, for example:
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The variables that act as directories (the root, animals, mouse, elephant, python, whatnot) are called hashes. Hashes store other variables (the so called subvariables) by a lookup name (e.g., "animals", "mouse" or "price").
The variables that store a single value (size, price, test and because) are called scalars.
When you want to use a subvariable in a template, you specify its path from the root, and separate the steps with dots. To access the price of a mouse, you start from the root and go into animals, and then go into mouse then go into price. So you write animals.mouse.price. When you put the special ${...} codes around an expression like this, you are telling FreeMarker to output the corresponding text at that point.
There is one more important kind of variable: sequences. They are similar to hashes, but they don't store names for the variables they contain. Instead, they store the subvariables sequentially, and you can access them with a numerical index. For example, in this data-model, animals and whatnot.fruits are sequences:
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To access a subvariable of a sequence you use a numerical index in square brackets. Indexes start from 0 (it's a programmer tradition to start with 0), thus the index of the first item is 0, the index of the second item is 1, and so on. So to get the name of the first animal you write animals[0].name. To get the second item in whatnot.fruits (which is the string "banana") you write whatnot.fruits[1].
Scalars can further divided into these categories:
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String: Text, that is, an arbitrary sequence of characters such as ''m'', ''o'', ''u'', ''s'', ''e'' above. For example the name-s and size-s are strings above.
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Number: It's a numerical value, like the price-s above. The string "50" and the number 50 are two totally different things in FreeMarker. The former is just a sequence of two characters (which happens to be readable as a number for humans), while the latter is a numerical value that you can use, say, in arithmetical calculations.
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Date/time: A date or time. Like the date an animal were captured, or the time the shop opens.
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Boolean: A true/false (yes/no, on/off, etc.) thing. Like animals could have a protected subvariable, which store if the animal is protected or not.
Summary:
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The data-model can be visualized as a tree.
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Scalars store a single value. The value can be a string or a number or a date/time or a boolean.
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Hashes are containers that store other variables and associate them with a unique lookup name.
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Sequences are containers that store other variables in an ordered sequence. The stored variables can be retrieved via their numerical index, starting from 0.