While the |theme filter returns an address relative to the active theme path of the website. The result is an absolute URL, including domain and protocol, to the asset specified in the filter parameter.
So |themel filter returns a relative file path without domain and protocol, to the asset specified in the filter parameter.
According to w3schools.com:
It is best practice to use relative file paths (if possible).
When using relative file paths, your web pages will not be bound to your current base URL. All links will work on your own computer (localhost) as well as on your current public domain and your future public domains.
{{ 'assets/svg/image.svg'|theme }} become:
https://domain.tld/themes/your-theme/assets/svg/image.svg
{{ 'assets/svg/image.svg'|themel }} become:
/themes/your-theme/assets/svg/image.svg
For img tag:
<img src="/themes/your-theme/assets/svg/image.png" alt="October">or link tag for <head> section:
<link rel="stylesheet" href="/themes/your-theme/assets/css/main.css" class="css">