Tokenised stocks have long been heralded as one of the more obvious bridges between traditional finance and crypto. And now, the US is finally starting to open that door.The SEC had recently introduced an Innovation Exemption, which gives certain platforms a five-year window to experiment with trading US stocks using blockchain technology. But what exactly does that mean?In simple terms, think of it like this. Instead of buying a share of Apple through the traditional stock market infrastructure, investors could eventually hold a digital token representing that very same share on a blockchain instead.The idea here is mainly to bring some of crypto's technology into traditional markets, potentially allowing for faster settlement and more flexible trading. That sounds straightforward enough, no?Well, not quite. The SEC is also putting some fairly strict guardrails around the experiment.The new framework comes with limits on how many stocks that platforms can offer and how much trading activity can take place. For example, platforms can initially offer up to 75 of the largest US stocks with trading capped at just 0.25% of each stock's average daily volume. Meanwhile, smaller stocks may get slightly more room but there are still restrictions regardless.And for Robinhood, they think that these limits could become a problem rather quickly.The firm's crypto chief, Johann Kerbrat, spoke to The Block and said that the company's existing stock token business outside the US is already seeing enough activity that it could bump against some of the SEC's thresholds. That is an important caveat to take note of.Now, we also have to put things into context here. Robinhood's current stock tokens are offered outside of the US and are structured differently from what the SEC is proposing. So, it isn't exactly an apples-to-apples comparison.Having said that, I would argue that the bigger point still stands.The SEC is clearly willing to let tokenised stocks develop in the US. However, it wants to do so in its own terms - which for now means slowly and under controlled conditions.That doesn't mean that things might not change moving forward though. SEC chair, Paul Atkins, has also described the exemption as a bridge towards more permanent rules rather than the final destination. So while they may be putting on the brakes for now, it doesn't mean that they won't take their foot off the pedal at some point in the future.But as we're starting to see here, the conversation is no longer centering on whether stocks can move onto blockchain technology. It is starting to shift towards the idea of whether regulators can keep up if investors actually start using them. This article was written by Justin Low at investinglive.com.