> For the complete documentation index, see [llms.txt](https://docs.workersforlabview.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.workersforlabview.io/fundamental-concepts/important-terms.md).

# Important Terms

A Workers application is built up out of one or more Workers, connected together in a call-chain hierarchy that looks like a tree. Lets take an example of an application that is built out of four Workers, whose Worker call-chain hierarchy is represented by the diagram below.

![An application's Worker call-chain hierarchy](/files/OiNn3ra0Lnq1FKXo2Biw)

## Head Worker

Every Workers application has a Head Worker. An applications Head Worker is the Worker at the very top of an application's Worker call-chain hierarchy. In the figure below, **Worker A** is the Head Worker of the application.

![Worker A is the head Worker of the application](/files/5PtrE9N16DPVpPYhNdat)

## Caller

A Caller is a Worker that calls (or loads) one or more Workers. In the figure below, **Worker A** is the Caller of Workers B, C and D.

![Worker A is the Caller of Worker B, C and D.](/files/5PtrE9N16DPVpPYhNdat)

## subWorker

A subWorker is a Worker that is called (or loaded) by another Worker. In the figure below, **Worker B, Worker C and Worker D** are subWorkers of Worker A.

![Worker B, C and D are subWorkers of Worker A](/files/kI3cfoRlSeBCkfq0nvez)

## Worker Main Data Wire

A Worker's Main Data Wire is the Worker class wire that starts at the Worker's object constant and runs through the Worker's MHL. See the image below.

<figure><img src="/files/c7QVHDEEc59zo9OHfzor" alt=""><figcaption><p>Worker Main Data Wires and Worker Handles</p></figcaption></figure>

## Worker Handles

Worker handles are class wires that contain a Worker's Public Properties (such as the Worker's Message Queue). Worker handles are used to send messages to a specific Worker instance via the Worker's Public Request VIs. An example of a Worker's handle could be:

1. subWorker class wires that are used by a Caller's MHL (such as in the image above).
2. The Worker class wire that exits an [Asynchronous Launcher VI](/fundamental-concepts/launcher-vis.md#asynchronous-launcher-vis).
3. The Worker class wire that exits a [Dynamically Load Worker Public API VI](/workers-tools/workers-tools-menu/public-api-builder.md#create-dynamically-load-worker-vi).