For Free/Opensource documentation, There are two license widely used.
Both say re-distribution must be under same license as GFDL is copyleft and CC-BY-SA is with share-alike attribution.
So, I want to know whether they are compatible with each other or not? Further which should be used when?
From the Creative Commons Attribution-ShareAlike 3.0 legal code (version 4.0 has similar wording):
You may Distribute or Publicly Perform an Adaptation only under the terms of:...(iv) a Creative Commons Compatible License.
"Creative Commons Compatible License" means a license that is listed at https://creativecommons.org/compatiblelicenses that has been approved by Creative Commons as being essentially equivalent to this License
From the above link:
Currently, no non-CC licenses have been designated as compatible with BY-SA 3.0
Currently, the Free Art license 1.3 is the only “BY-SA–Compatible License” for version 4.0.
So no, you cannot incorporate content from a CC-BY-SA work into a GFDL work.
Going the other direction is even more restrictive. From section 4 of the GFDL 1.3:
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License
It's quite explicit that the only permissible license for a derivative work of a GFDL document is the GFDL. Section 11 (relicensing) doesn't change anything: it's essentially the "Wikipedia clause", included to permit the Wikimedia Foundation to change from the GFDL to CC-BY-SA.
In general, if you are interested in creating genuinely open content, the GFDL should not be used. The mechanics of complying with it are extremely unwieldy: in particular, the requirement to include a copy of the 23KB of license text make it impractical for anything but electronic content or book-sized printed works.
