There is no Turnitin similarity score threshold below which an assignment is considered not plagiarized; rather, this depends on the type of match found. For example:
A 20% Turnitin similarity score due to the report template, questions copied from the submission requirements, or technical definitions that cannot be changed, this is not considered plagiarism.
A 20% Turnitin similarity score is considered plagiarism if this stems from an entire paragraph copied and pasted from a document that you don’t own.
A submitted assignment is not just a collection of quotes. If you use a statement copied verbatim from a source, you must use quotation marks and add proper citation. In addition to that, you must explain the reason why you copied that statement and why it supports your personal reasoning (i.e. Does it prove or disprove what you are trying to say? How is this quote guiding your personal statements and research?). The point of using a quotation from a source is to support the argument you are making and should NOT become the argument itself.
Although there is no specific rule on the length of a quoted statement, there is no need to copy an entire paragraph. Should the instructor wish to read the quote at length, they may do so by referring to the original source that you must cite where you place the quote and must include in your references/bibliography. If the copied block is too long and/or it is not cited, expect to be heavily penalized for plagiarism even if it is quoted and referenced.
At this time, Turnitin has only made AI detection scores accessible to instructors. Students cannot review this score.