Welds shall be terminated at the end of the joint in a manner that will ensure sound welds. Whenever necessary, this shall be done by use of weld tabs, also called extension bars and runoff tabs. Weld tabs shall extend beyond the edge of a joint a distance equal to a minimum of the part thickness, but not less than one inch - https://goo.gl/iw1MZK - page 4-2
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Applicable reference:
-AWS D1.5 (02) Section 3.12.1 – “Welds shall be terminated at the end of a joint in a manner that will ensure sound welds. Whenever possible, this shall be done by use of weld tabs (extension bars and runoff plates) placed in a manner that will duplicate the joint detail being welded.” - http://www.dot.ca.gov/pra/NCR/Nonconformance_Report_document... -page 2

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To produce sound welds on many welded joint geometries, run-off tabs projecting from the finished member may be required to permit starting and stopping welds beyond the edge of the member; AWS D1.1:2004 Sections 5.10 and 5.31 should be followed. When such welding aids are required to be removed, the surface should be finished as indicated in 2.2.6 and 2.2.7 - https://www.aisc.org/steel-solutions-center/engineering-faqs...

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Tack weld starter/run-off tabs or groove extensions at each end of the joint. The starter/run-off tabs should be constructed of ¼" / 6mm minimum thickness mild steel, (ASTM A36 or equivalent) and must extend at least 2" / 50mm beyond the ends of the weld grooves. - http://www.hensleyind.com/wp-content/uploads/2017/10/welding... - page 23

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This video is for people looking for information about using run off tabs for welding. - https://www.youtube.com/watch?v=j1Zh1Uz0DfQ