In short :  ideally a bounded context is the model of a sub-domain but this cannot always apply when for instance a technical requirement for a sub-domain conceptually requires a bounded context for itself (the provided example was a full-text search feature in a document management domain).

Summary

A sub-domain delimits a domain and exists within the problem space. A bounded context delimits the domain model and exists within the solution space. The ideal is full alignment between a sub-domain and a bounded context, however in practice a degree of flexibility must be accepted in this regard. Furthermore, just as the bounded context is guided by a sub-domain, a sub-domain is, in turn ,guided by a bounded context as part of the model exploration whirlpool.

source :  http://gorodinski.com/blog/2013/04/29/sub-domains-and-bounded-contexts-in-domain-driven-design-ddd/