Mlogit软件包：“变化”到底代表什么？

Question

我从网络上获得了以下脚本，但我不知道如何解释variant = 3：6。有人可以帮我吗？

 list.of.packages <- c("mlogit", "data.table")
install.packages(list.of.packages[!(list.of.packages %in% installed.packages()[,"Package"])])
sapply(list.of.packages, library, character.only = TRUE)



cbc.df <- read.csv("http://r-marketing.r-forge.r-project.org/data/rintro-chapter13conjoint.csv", colClasses = c(seat = "factor", price = "factor" ))
summary(cbc.df)

cbc.mlogit <- mlogit.data(data=cbc.df, choice="choice", shape="long", varying=3:6, alt.levels=paste("pos", 1:3), id.var="resp.id")
cbc.ml <- mlogit(choice ~ 0 + seat + cargo + eng + price, data = cbc.mlogit)
summary(cbc.ml)