Several parameters in various macros support being injected using a "%key:value%" syntax, also known as Supplier Keychains.
Parameter Injection
The parameters which support this are marked as such in the documentation. This allows you to inject Supplier values into the parameter, based on whatever the context of the macro is in.
For example, the Report Variable macro uses this for the 'value' and 'default' parameters, like so:
{report-variable:Parent Name|value=%page:parent > page:title%}
You can also use this multiple times, or within the context of other text, in which case it will result in a Text value, as shown in the following example:
{report-variable:Message|value=Created by %page:creator > user:name% on %page:creation date > date:dd-MMM-yyyy%}