Turnitin is a plagiarism tool that makes text matches from other documents in the repository of papers, but it does have some limitations of what it will make matches with.

Turnitin is billed as a plagiarism tool. This means it is a tool that can aid in detecting plagiarism by detecting text matches in documents accessed in the student repository, as well as other sources. When a text match is found, it is highlighted in the document and provides data to the file they believe the information is from. Keep in mind it is up to you as the instructor to evaluate these matches and determine if they have quoted and referenced the material properly or if the match is actually a misuse of the text.  

What is an acceptable similarity report percentage?

Again this is up to you as an instructor. There is no rule that states plagiarism is acceptable if the similarity report scores below a certain percentage.

Each case must be evaluated by you as the instructor. Turnitin makes the process easier simply by identifying text matches for you, and letting you accept or reject the matches.

Can Turnitin detect all aspects of a document?

No, Turnitin cannot detect, display or process hidden content, linked documents or documents that contain multiple files such as PDF portfolios.