Please explain. I marked it in a red circle.
It's rounded in red color.I got this from the book Computer networking a top down approach.Pg NUM 241. Section 3.4 Rdt 3.0
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The capital lambda 'Λ' is the "output alphabet" -- it's part of the notation for finite state machines, and is the set of possible outputs. The trigger events of the transition come from the "input alphabet" (normally capital sigma 'Σ'). Transition events are listed above the line, the actions underneath.
See for example http://www.dcs.ed.ac.uk/home/mic/FiniteStateMachines1-slides.pdf
Or, on the subject of reliable data transmission http://www.nyu.edu/classes/jcf/CSCI-GA.2262-001_fa13/slides/session6/ReliableDataTransfer.pdf