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For Free/Opensource documentation, There are two license widely used.

  1. GNU Free Documentation License
  2. Creative Commons BY-Share Alike

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?

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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.

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