Legal Question in Wills and Trusts in California

The succesor trustee is the the trustee that comes next. The succesor trustee can serve by him or herself, and then be sole trustee. The settlor is who sets up the trust, and may or may not be the first trustee.

Sole (only) trustee just means only one person is serving as trustee, as opposed to co-trustees or multiples trustees. A sole trustee could be the original settlor or a successor trustee.

I would add that the settlor, sometimes referred to as the trustor, is the person or persons who establish the trust and whose property becomes the property (also called the corpus) of the trust. Most of the time, the settlor is his/her/their initial trustee of his/her/their own trust. The successor trustee or trustees are the individual/indiviuals who the settlor has nominated to assume the duties when the original trustee is no longer able or unwilling to act as such. As the other attorneys have said, a sole trustee just means that there is (or will be) only one acting trustee.