Sentence:  Enhanced Data Rates for Global Evolution (EDGE), also known as Enhanced GPRS (EGPRS), is a digital mobile phone technology which acts as a <WSD>bolt-on</WSD> enhancement to 2G and 2.5G General Packet Radio Service (GPRS) networks. 
Meaning of the word marked with <WSD> tags:  Used to extend an existing system, operating as an add-on or plug-in.