[kgr]

3♣=6c♣; intermediate (10-15 HCP)
From our system: "Fit-bid: When opps compete then we play for responder splinters in suits promised by the opps and fit-bids in other suits. A jump by Opener in competition is a splinter."
It says nothing about advancer.

(1♠)-3♣-(Pass)-4♦
=> Is it best to have 4♦ here as fit-bid or as splinter?
=> ...and 5♦?

[Fluffy]

fit bid, 5♦ should be natural.

[TWO4BRIDGE]

kgr, on 2013-September-06, 10:40, said:

3♣=6c♣; intermediate (10-15 HCP)
From our system: "Fit-bid: When opps compete then we play for responder splinters in suits promised by the opps and fit-bids in other suits. A jump by Opener in competition is a splinter."
It says nothing about advancer.

(1♠)-3♣-(Pass)-4♦
=> Is it best to have 4♦ here as fit-bid or as splinter?
=> ...and 5♦?

I have in my files ( sorry, no link ) from 2004 regarding Rubens Transfer Advances .

He defines the following "Jumps" by Advancer :

a ) A jump-shift is a fit-bid in support of overcaller's suit
...... ( 1C ) - 1H - ( p ) - 2S/3D = fit-showing-jumps

b ) A jump-cue-bid to the 3-level ( ie. a single-jump ) shows the values for a constructive raise ( 7-9 hcp) with four or more trumps.
...... ( 1C ) - 1H - ( p ) - 3C!

c ) A jump-cue-bid to the 4-level ( ie. a double-jump ) shows shortness, support and a slam try ... 4+ trumps
...... ( 1C ) - 1H - ( p ) - 4C! = splinter .

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kgr : Now, regarding your auction, if the overcall were 2C, then 3D would be a fit-showing jump and 4D would be a splinter.
But, since your overcall is 3C [ 6c♣; intermediate (10-15 HCP) ] , you may have to redefine to whatever you agree with partner .

[kgr]

TWO4BRIDGE, on 2013-September-07, 09:11, said:

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kgr : Now, regarding your auction, if the overcall were 2C, then 3D would be a fit-showing jump and 4D would be a splinter.
But, since your overcall is 3C [ 6c♣; intermediate (10-15 HCP) ] , you may have to redefine to whatever you agree with partner .

I think it will be most logic to have:
- Any bid is forcing (except 4♣)
- 4♦: fit-bid
- 4♥: to play
- 4♠: splinter
- 5♦: to play