seal在法律文件中一般有2层含义：

1. 盖章，如signature and seal 签字盖章

2. 密封，如to seal a letter 封上信，多见证明或者专利文件

1例句

This contract shall come into force after signature and seal by the buyers and sellers.

本合同需经买卖双方签字盖章后生效。

2例句

The Submersible Pump and Motor have a double seal with a moisture detection system. 

配置潮气监测系统的潜水泵和电机具有双重密封装置。