Level 1 -

Openhole sidetrack or unsupported junction.
Level 2 - Cased and cemented main wellbore with openhole lateral or dropoff liner.
Level 3 - Cased and cemented main wellbore with uncemented lateral liner mechanically connected to the main well-bore (red). Level 3 junctions incorporate a liner tieback and mechanical connection to the primary casing that permit selective access reentry to lateral branches.

Level 4 -

Cased and cemented main wellbore with cemented lateral liner mechanically connected to the main wellbore.
Level 5 - Cased and cemented main wellbore and uncemented or cemented lateral liner with hydraulic and pressure integrity provided by additional completion equipment inside the main wellbore—packers, seals and tubulars.
Level 6 - Cased and cemented main wellbore and uncemented or cemented lateral liner with hydraulic and pressure integrity provided by the primary casing at the lateral liner intersection without additional completion equipment. Level 6 junctions are an integral part of the primary casing string that provides pressure integrity and lateral reentry access.

Junction Classifications
Multilateral junctions are classified as Level 1 through 6 according to the Technical Advancement of Multilaterals (TAML) Forum held in Aberdeen, Scotland, July 26, 1999 and recently updated in a July 2002 draft proposal.

There are two broad groups based on mechanical integrity and hydraulic isolation: those that do not provide pressure integrity (Level 1, 2, 3 and 4), and those that do (Level 5 and 6). Level 3 and Level 6 systems have emerged as preferred multilateral junctions.