Promoted attributes
Promoted attributes are attributes which are considered important and thus are "promoted" onto main note UI. See example below:
You can see the note having kind of form with several fields. Each of these is just regular attribute, the only difference is that they appear on the note itself.
Attributes can be pretty useful since they allow for querying and script automation etc. but they are also inconveniently hidden in the note attributes dialog. This allows you to select few of the important ones and push them to the front of the user.
Now, how do we make attribute to appear on the UI?
Attribute is always name-value pair where both name and value are strings.
Attribute definition specifies how should this value be interpreted - is it just string, or is it a date? Should we allow multiple values or note? And importantly, should we promote the attribute or not?
In the above picture you can see two labels - tag and todoDate with some values. But below them you can notice again tag and todoDate attributes, but now of type "Label definition". These "definition" attributes define how the "value" attributes should behave.
So there's one attribute for value and one for attribute. But notice how definition attribute is Inheritable, meaning that it's also applied to all descendant note. So in a way, this definition is used for the whole subtree while "value" attributes are applied only for this note.
Some relations always occur in pairs - my favorite example is on the family. If you have a note representing husband and note representing wife, then there might be a relation between those two of isPartnerOf. This is bidirectional relationship - meaning that if a relation is pointing from husband to wife then there should be always another relation pointing from wife to husband.
Another example is with parent - child relationship. Again these always occur in pairs, but in this case it's not exact same relation - the one going from parent to child might be called isParentOf and the other one going from child to parent might be called isChildOf.
Relation definition allows you to specify such "mirror relation" - for the relation you just define you specify which is the mirror relation. Note that in the second example we should have two relation definitions - one for isParentOf which defines isChildOf as mirror relation and then second relation definition for isChildOf which defines isParentOf as mirror relation.
What this does internally is that whenever we save a relation which has defined mirror relation, we check that this mirror relation exists on the relation target note. Similarly when we delete relation, we also delete mirror relation on the target note.