The legal framework as well as the technical implementation for seals are in most respects identical to signatures. The main difference between the two is that signatures implies legal intent on behalf of the of the signatory, which is not the case for seals.
The three levels of signatures, SES, AdES and QES, exist for seals as well and are somewhat confusingly named the same, namely Simple Electronic Seal (SES), Advanced Electronic Seal (AdES) and Qualified Electronic Seal (QES).