What are these IDA fields?

Can someone confirm what are these _ida fields that we sometimes see in vardefs?

For example I created a relationship between leads and users (1-to-N) and now seeing the following:

$dictionary["User"]["fields"]["leads_users_1leads_ida"] = array (
'name' => 'leads_users_1leads_ida',
'type' => 'link',
'relationship' => 'leads_users_1',
'source' => 'non-db',
'reportable' => false,
'side' => 'right',
'vname' => 'LBL_LEADS_USERS_1_FROM_USERS_TITLE',
);

and above it there's a very similar definition but without IDA

$dictionary["User"]["fields"]["leads_users_1"] = array (
'name' => 'leads_users_1',
'type' => 'link',
'relationship' => 'leads_users_1',
'source' => 'non-db',
'module' => 'Leads',
'bean_name' => 'Lead',
'vname' => 'LBL_LEADS_USERS_1_FROM_LEADS_TITLE',
'id_name' => 'leads_users_1leads_ida',
);