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Direct or Standard

There are two types of deraileur, and the deraileur specs usually list which type. But the terminology is quite confused, so let's start at the beginning. Derailleurs attach to bikes via a 10mm metric bolt, referred to as the "M10" bolt. The threaded hole for this bolt is on the derailleur "hanger" of the bike frame. On older bikes, the derailleur hanger was a permanent part of the frame. With newer frames a separate hanger is bolted onto the frame, and then the derailleur is bolted onto the hanger.

Derailleurs can be either "direct mount" or "standard mount". The words are confusing, for now just know there are two types. A direct mount derailleur can be directly attached to a direct mount hanger. A direct mount hanger is longer and projects about 2cm further back than the "standard" hanger. A direct mount derailleur often comes with a B link. You can see the B-Link in the photo, there are two 10M bolts about 2 0r 3 cm apart. I think the "B-Link acts as some kind of adapter, that allows you to use a direct mount derailleur on a standard mount frame. By using the B-Link, you can mount it onto a standard hanger. By removing the B link, you can mount it directly on a direct mount hanger.

A "standard" derailleur can only be mounted on a standard hanger.
  With direct mount, the derailleur is about 2 cm further back from the axle, whereas the standard amount is is straight down.

In theory, you could also switch the hanger for a given frame. But this is rare. The interface between hanger and frame is not standard, even within a given manufacturer like Trek. The replacement hangers for Trek and Salsa are for "standard" type derailleurs, not "direct".

Direct mount is said to be rare on touring bikes. However, the Alivio M4000 which is said to fit touring bikes is listed as "Direct Mount". So how does that reconcile? The picture shows it has a B Link. So I presume the way to hook it up is by using the B-Link. If you were using it on a direct frame, then you would remove the B Link, and directly M10 bolt it to hanger.

See this article for a reference: https://wheelsmfg.com/blog/standard-mount-vs-direct-mount-derailleur-hangers.html