插入符号火车功能输出？

Question

考虑到R在插入符号包中对train函数的以下用法：

model <- train(
    medv ~ ., 
    Boston,
    method = "lm",
    trControl = trainControl(
      method = "cv", 
      number = 5,
      verboseIter = TRUE
    )
  )
+ Fold1: intercept=TRUE 
- Fold1: intercept=TRUE 
+ Fold2: intercept=TRUE 
- Fold2: intercept=TRUE 
+ Fold3: intercept=TRUE 
- Fold3: intercept=TRUE 
+ Fold4: intercept=TRUE 
- Fold4: intercept=TRUE 
+ Fold5: intercept=TRUE 
- Fold5: intercept=TRUE 
Aggregating results
Fitting final model on full training set

1）请告知+和-FoldX是什么意思？

+ Fold1: intercept=TRUE 
- Fold1: intercept=TRUE

2）以下是否表示这是所有5折的平均RMSE：

Resampling results:

  RMSE      Rsquared   MAE     
  4.865282  0.7230966  3.389431

Tuning parameter 'intercept' was held constant at a value of TRUE