Several parameters in various macros support being injected using a %key:value% syntax, which are called Supplier Keychains.
The parameters which support this are marked as such in the relevant macro documentation page. 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 (from REP) uses this for the Value and Default Value parameters, like so:
You can also do 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:
Where the full Value is:
Created by %page:creator > user:name% on %page:creation date > date:dd-MMM-yyyy%