What is being searched with these different options varies depending upon the type of data that you are searching in. 

For “title and abstract only”, the keywords will be searched for in the title and abstract fields only, when searching in grants or publications. When searching in clinical trials and patents, this will search the title and description/summary for each record.

For “full data” searches, the keyword will be searched for in the full text of the article, where this is available.