When no search operators like "AND" or quotes are used, all results that include any of the terms are returned, along with those that contain all terms the greatest number of times in important fields displayed at the top.
The total number of results does not affect the relevance of the ones shown at the top: '"AND"ed results display higher than '"OR"ed results. Using quotes and Boolean operators will alter which items are considered most relevant.
Both SpringerLink and Springer for R&D support the AND, OR, and NOT Boolean operators, as well as the ? and * wildcards. Boolean operators need to be typed in all caps (e.g. “cardiac AND thoracic”) to be recognized.