Speaking to a room full of CEOs at a summit, the CEO of Chegg once said "If you are raising a series F, it means what you think it means."
Tesla raised Series F in 2009, and it's still alive.
They're presently worth $42 billion with $2.1b in sales, 172% sales growth over three years, and are likely set to be another enterprise software juggernaut.
It clearly doesn't inherently mean anything. What matters is whether the business is growing properly versus the capital being consumed. If more capital will reasonably accelerate growth further, feed the business more capital (depending on what the owners want to pursue of course).
Is there a break down somewhere of what each series should signify?