Non-Favoured Maps found in your Maps drop as Basic Currency Items instead
(Basic Currency includes any currency that drops from non-league content) 
Triples the weighting of Favoured maps; the number of times the map is Favoured does not affect this (other than indirectly increasing the weighting). It is best used to oversustain a single map. It may be better to Favourite more than one map for overall map sustain. Note that this keystone does not triple the amount of maps dropped; due to the loss of Map Equity through the currency conversion, one may find less maps with the Keystone than without it.
This does not affect certain Map Rewards such as those from Delirium rewards.
Singular Focus will prevent the player from dropping Unique Maps (including Synthesis Maps), Shaper Guardian maps, and Elder Guardian or Conquerors maps that generate with a base map other than your Favoured maps. Elder Guardian and Conquerors maps that generate with Favoured base maps will still appear.
Blighted maps also can only appear if they generate on your Favoured base map, and not all base maps have a corresponding Blighted version.
As of version 3.18.0 it is located near the center of the atlas passive skill tree, to the upper right of the central "Kirac" wheel.