What is the difference b/w gl_je_headers.default_effective_date and gl_je_lines.effective_date

What is the difference b/w gl_je_headers.default_effective_date and gl_je_lines.effective_date can these two dates vary in a jv's line. Like a heders date is 01-jan-2011 and in lines date is 01-jan-2011 , 02-jan-2011 and some other, is it possible ? if yes then what is the scenario. Also the posted status in headers and lines can these status be vary in headers and lines if a jv is posted in headers than is it possible for a jv to have both unposted lines too in it ?

Thanks & Regards

Aqeel