What is a settlor?
The settlor is the person who creates a trust and transfers assets into it. Once transferred, the assets generally leave the settlor's ownership — though anti-avoidance rules can still tax the settlor on the trust's income or gains where they or their spouse retain a benefit.
A settlor can also be one of the trustees, and often is. What they cannot usually do is retain a benefit without consequence: a settlor-interested trust is taxed on the settlor, and a gift into a trust from which the settlor still benefits is a gift with reservation of benefit for inheritance tax, so the seven-year clock never starts.
The settlor's active role largely ends when the trust is created. From that point trustees run it. The settlor's continuing influence, such as it is, runs through the letter of wishes.
Where more than one person contributes assets, there can be more than one settlor, and each is tracked separately for tax.