I am pretty sure the superclass and subclass terminology from S4 classes and general programming lingo apply to S3 classes too. See R Class Definitions for details.
For example:
library(dplyr)
df <- tibble(
x = c(1,2),
y = c("a","b")
)
df
class(df)
getClass('tbl_df')
Returns
> df
# A tibble: 2 x 2
x y
<dbl> <chr>
1 1 a
2 2 b
> class(df)
[1] "tbl_df" "tbl" "data.frame"
> getClass('tbl_df')
Virtual Class "tbl_df" [package "tibble"]
Slots:
Name: .Data names row.names .S3Class
Class: list character data.frameRowLabels character
Extends:
Class "tbl", directly
Class "data.frame", by class "tbl", distance 2
Class "list", by class "tbl", distance 3
Class "oldClass", by class "tbl", distance 3
Class "vector", by class "tbl", distance 4
Known Subclasses: "grouped_df", "rowwise_df"
So, yeah, myclass is a subclass of data.table and data.frame. And data.frame is the superclass of data.table.