Any multimodal composition is about communication. Ultimately, you are writing (or composing) for a specific audience. That audience might be the general public, an academic discipline, a professional community, or even a fandom or other interest group. Understanding and analyzing the language, publication standards and common mediums, tone, and communication styles of your audience can help you to compose in way that better communicates your desired message. You will have to make decisions along the way based around what mediums and modes you use, the tone you will write in, your documentation style (for citations), and an understanding of your audience will help you with all of these. It can be useful to look at examples of compositions from your audience community to see what the standards are.

According to Arola, Kristin L, Jennifer Sheppard, and Cheryl E. Ball, communication modes include:

Linguistic - This mode is concerned with words. What words you write with, how you form your sentences and paragraphs using words together, and how the text or words are delivered are all components of the linguistic mode. While it is the most commonly used, it may not be the most important mode in all contexts. The linguistic mode will work in conjunction with other modes.

Visual - This mode is about images and the visual appearance of text. The visual mode refers to both actual images, artwork, and designs but also the visual layout, text size and font, emboldened or italicized text, and the color of the background and text. 

Aural - This mode is about sound. The aural mode can be music, sound effects, or the absence of sound, but it also includes human speech and aspects of sound, like tone, speed, volume, emphasis, voice, etc.

Spatial - Refers to physical organization and arrangement of text and other information. This might be how text is placed on pages in a book, but it could be how a webpage and website are navigated. 

Gestural - This mode is concerned with movement. Usually gesture is discussed in the context of facial expression, posture, and other body movements.