ASE

These experience shared by  Senior DBAs as name mentioned, Hope this  will help you to understand more about function string from implementation point of view in a Replication environment: 
Craig Oakley , Senior DBA.
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We used function strings when we wanted to replicate all columns to some servers, and only selected columns to other (web-facing) servers. This was particularly useful before Rep Server allowed multiple RepDefs on the same table. One concern was text columns which were not being replicated to the web-facing server: we had to create a function string to get a text pointer (we used a one-row table and just update all the text columns on top of each other, as the value was not needed on that server): failure to get a text pointer cause the DSI to go down, and we could not specify that as a condition to ignore.

Beyond this, I would imagine function strings could help specify how you want the update to be done, which could be a performance improvement. It would also allow for a different implementation at the replicate than there is at the primary (such as a table at the primary being two joined tables at the replicate).

Sukhesh Nair, Senior Sybase DBA
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We used to have a setup where data was replicated from sybase to oracle as also to a warm standby sybase server. Rep Server function strings helped in filtering data that would need to be passed to Oracle. It helped immensely in streamlining the data flow to targets by manipulating the incoming data through function string. I feel it is one of the most advanced and useful yet very less used capabilities of Sybase Rep Server.
The deterrent could be because of the complexity it would introduce to the replication system. The setup we had worked wonderfully and never gave us any major problems. Without proper monitoring (which needs to be scripted by DBAs) it used to be hard to maintain. Many of the current Rep Server administrators I see do not have adequate knowledge or experience of handling function strings.

Rey Wang , Senior Sybase DBA
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You can map the delete to no op with functional string.

Partha Gogoi Senior DBA
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We use function strings to transform data at the replicate..We have databases being replicated from Toronto and New York to London, Sydney and Singapore and the client ids are transformed at the replicate because, as per business requirements, the client ids are different at each site.. Of course , having a Universal client id would simplify things , but the systems and databases at each site grew independently until replication was set up and it would be a lot of rework to change all the client ids at the replicate sites

Øystein Grinaker Senior DBA
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A Function String could be used to change default behaviour.
Say you delete a row in a table on PDB, but you do not want to delete the row on the RDB. Then make a change in rs_delete. You may make the rs delete just to make a logical delete by updateing a deletemarker for that spesific row.

Source : Linkedin.com

http://www.linkedin.com/groups/What-are-possible-usage-Function-3955841.S.65585220?qid=4eb177c2-d7e3-4572-9be4-92d640046c6c&trk=group_most_recent_rich-0-b-ttl&goback=%2Egmr_3955841