In the IBC technical article GL-30 Betta Variations - Normal vs Adnormal, Dr. Gene Lucas has this to say about the use of these symbols in Betta genetics.

"Basically the system is quite simple. A variant is compared to the standard or wild type.It is described specifically in terms of how it varies. It is given an identification (a symbol) which, if possible, allows a quick identification of what it does to change the individual possessing it. "w" for example stands for white eye. Upper and lower case letters are employed to represent dominance or recessiveness of the gene symbolized. The effect is compared to the normal or wild type gene. Abnormal refers to "not normal" but does NOT mean harmful or inferior (unless it happens that the effect IS harmful, etc.) A deviation from the "old" method of using upper and lower case letters for two forms of a gene (allees) such as a "W" for the normal eye color in the example of white eye was introduced. A "+" symbol is used to designate the normal allee. It may then be contrasted to an abnormal one and iterpreted using the other symbol as a base. Symbolism such as W/w can be replaced by +/w. The lower case of the letter indicates that it is recessive to the normal. This frees a symbol, the W, to be used to designate some other variation. If it is used it will presumably be for some descriptive word which starts with a W and the genetic factor it represents would be indicated as dominant. Thus, +/W (or W/+) always indicates that the abnormal form of the gene is dominant.

This system permits, potentially, a doubling of the available symbols. At the same time, the symbolism actually yields more information. If one examines symbols such as W/w there is no way to know which form is normal, a bit of information which is often of concern. The use of +/w or +/W reveals immediately which is which. The recommendation to change symbols for the gene(s) responsible for the heavy irodocyte (metallic) color from the originally published "reduced iridocytes" (Edberhardt, 1941) to "Spread iridocytes", for example, represents the application of the more suitable and useful form of symbolism."

Streaked colors in fins
Also includes butterfly trait

Halts red pigment production at the orange stage
*not verified

Controls iridocyte color
BlBl=Green Blbl=Royal
blbl=Steel

Causes a creamy deposit
on fins and body

Variable color or lack
of on body and fins

2 tail lobes and usually
an extended dorsal

Incomplete Dominence

Caudal has 180deg spread
Anal fin increased size

Unknown
Possibly "Multiple Factor Inheritance"