The --v parameter selects which Midjourney model version to use. Always use the latest for best results.

Key Versions

• --v 6.1: Current default — best overall quality, most photorealistic
• --v 6: Great quality, slightly different aesthetic than 6.1
• --v 5.2: Still excellent, different style characteristics
• --niji 6: Specialized for anime and manga illustration styles

When to Use Older Versions

Sometimes older versions produce a specific aesthetic you prefer. Experiment — different versions have different "personalities."

Reference:

Midjourney model versions

https://en.wikipedia.org/wiki/Special:Search?search=Version%20Parameter