What does either of the appreviations b and u refer to

Question

In https://wiki.openstreetmap.org/wiki/Overpass_API/Overpass_API_by_Example

there is an example as shown below in the code section. I would like to know what is

b
u 
and b.u?

Code:

[timeout:600];
rel[admin_level=2][wikidata]->.a;
foreach .a -> .b (
  rel.a(if:t[wikidata] == b.u(t[wikidata]))->.d;
  rel.d(if: id() == d.min(id()) && d.count(relations) > 1 );
  convert rel ::id = _.u(id()),
             wikidata = _.u(t[wikidata]),
             all_relation_ids = d.set( id() );
  (._ ; .result; ) -> .result;
);

.result out;

Answer 1

These are just arbitrary names for sets. Sets contain OSM elements such as nodes, ways and relations. If you don't specify an explicit set name then the default set named _ will be used.

From your example: Set a is populated by this line:

rel[admin_level=2][wikidata]->.a;

Then it will loop through set a and populate set b:

foreach .a -> .b (

Inside the loop, a set d gets created:

rel.a(if:t[wikidata] == b.u(t[wikidata]))->.d;