ISOs have a set of specific qualities that must be present to be treated as ISOs. Most established companies issuing options will be NQSOs. RSUs are a lower volatility version, but with less flexibility in terms of timing (I can't delay "exercising" my RSUs in terms of timing when I recognize the income).

You can make ISOs be treated approximately as NQSOs, but otherwise, you're generally getting from an established program and within a company, you don't have to be prepped to negotiate one type vs the other. Over time, the company will no longer be able to issue ISOs and may implement other programs, but they will tend to be "one type fits all" in general.