Anywhere in a macro lambda list where a parameter name can appear, and where ordinary lambda list syntax (as described in Section 3.4.1 (Ordinary Lambda Lists)) does not otherwise allow a list, a destructuring lambda list can appear in place of the parameter name. When this is done, then the argument that would match the parameter is treated as a (possibly dotted) list, to be used as an argument list for satisfying the parameters in the embedded lambda list. This is known as destructuring.
Destructuring is the process of decomposing a compound object into its component parts, using an abbreviated, declarative syntax, rather than writing it out by hand using the primitive component-accessing functions. Each component part is bound to a variable.
A destructuring operation requires an object to be decomposed, a pattern that specifies what components are to be extracted, and the names of the variables whose values are to be the components.
18.104.22.168.1 Data-directed Destructuring by Lambda Lists
22.214.171.124.2 Lambda-list-directed Destructuring by Lambda Lists