The bottom method attempts to return only one value but if the search is not specific enough it will return an error stating there was more than one item found.
Example in C#
// Returns: T
var item = await client.SearchOneAsync<T>(FolderInfo folder, String query, Boolean includeChildFolders, Boolean spanScope, Boolean includeParents, String collationLocale, Int32 collationStrength, String hints, Boolean useTwoStageQuery, Int32 firstStageCandidateLimit);