Chapter 71 justified the decision, chapters 73-76 build the actual logic - this chapter in between handles the pure registration: the one declaration that lets Magento know the type code loyalty_points_package exists at all.

product_types.xml as a configuration type

etc/product_types.xml is itself technically an example of what chapter 88 (block 11) introduces more generally as a configuration type: an XML structure merged across modules, read once, and cached afterwards - not a database value like System/Config/Setting (chapter 7). In practice, for this chapter that mainly means one thing: changes to this file need an explicit cache flush, more on that in a moment.

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Catalog:etc/product_types.xsd">
    <type name="loyalty_points_package"
          label="Punkte-Paket"
          modelInstance="Mironsoft\Loyalty\Model\Product\Type\PointsPackage"
          indexPriceModel="Magento\Catalog\Model\ResourceModel\Product\Indexer\Price\DefaultPrice"
          priceModel="Mironsoft\Loyalty\Model\Product\Type\PointsPackage\Price"
          isQty="true"
          canUseQtyDecimals="false"
          sortOrder="110"/>
</config>

The attributes in detail

• name - the type code itself, from here on identical, character for character, to the constant PointsPackage::TYPE_CODE (chapter 73).
• modelInstance - the type model class chapter 73 implements next; without it, every action against this type code immediately fails with an exception.
• priceModel - the class responsible for price calculation, also chapter 73.
• indexPriceModel - deliberately the same default class used by simple/virtual (DefaultPrice), since a points package needs no tier- or configuration-dependent price logic that would justify a dedicated price indexer - relevant for chapter 77.
• isQty - a quantity field is shown in the cart ("buy 3 points packages"), same as simple/virtual.
• canUseQtyDecimals - deliberately false: a fractional quantity like "1.5 points packages" is not a meaningful business concept.
• sortOrder - 110, deliberately well above the core types (simple = 10, virtual = 20, etc.), so "Points Package" recognizably sits at the end of the "New Product" dropdown instead of squeezing between the core types.

Not functional yet

With just this one file, "Points Package" already shows up in the "New Product" dropdown - but clicking it immediately ends in a PHP exception, because Mironsoft\Loyalty\Model\Product\Type\PointsPackage and ...\PointsPackage\Price don't exist yet. Chapter 73 supplies exactly these two classes.