V1
TRIM(" [ ref:" + LEFT( $Organization.Id, 4) + RIGHT($Organization.Id, 4) +"."+ LEFT( Id, 4) + SUBSTITUTE(SUBSTITUTE(SUBSTITUTE(Id, RIGHT( Id, 4), ""), LEFT( Id, 4), ""), "0", "") + RIGHT( Id, 4) + ":ref ] ")
V2 Case ref number used in Email communication.
"ref:_"&LEFT( $Organization.Id,5)&SUBSTITUTE(RIGHT($Organization.Id,10),"0","")&"._"&LEFT(Id,5)&SUBSTITUTE(LEFT(RIGHT(Id,10),5),"0","")&RIGHT(Id,5)&":ref"