What is &rel=1 for in youtube videos code?

[netchicken1]

When embedding youtube videos I cut out &rel=1, but does anyone know why its there? (BTW this link is for a GPS on the Iphone video - but its just an example)

<object width="425" height="355"><param name="movie" value="http://www.youtube.com/v/RY8OxHeNKfY&rel=1"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/RY8OxHeNKfY&rel=1" type="application/x-shockwave-flash" wmode="transparent" width="425" height="355"></embed></object>

[SuzyUK]

&rel=1 = true = it will show related videos when it's finished playing
&rel=0 = false = it won't

I presume the default is 'true' and that perhaps the <embed> (IE only) needs the default to be explicit.. though that's pure guesswrok :)