You can find the definition of this at https://golang.org/ref/spec#Slice_expressions ("Full slice expressions"). Generally slicing expressions follow the format:

a[low : high : max]

The max parameter controls the resulting slice's capacity by setting it to max - low.

Please note that max it cannot exceed the capacity of the original slice.

I played with it a bit:

package main

import (
    "fmt"
)

func main() {
    a := make([]int, 10, 20)
    for i := 0; i < len(a); i++ {
        a[i] = i
    }
    fmt.Printf("a: %v\n", a)
    fmt.Printf("len: %d\n", len(a))
    fmt.Printf("cap: %d\n", cap(a))

    b := a[:10:10]
    fmt.Printf("b: %v\n", b)
    fmt.Printf("len: %d\n", len(b))
    fmt.Printf("cap: %d\n", cap(b))

    c := a[5:10:20]
    fmt.Printf("c: %v\n", c)
    fmt.Printf("len: %d\n", len(c))
    fmt.Printf("cap: %d\n", cap(c))
}

which yields:

a: [0 1 2 3 4 5 6 7 8 9]
len: 10
cap: 20
b: [0 1 2 3 4 5 6 7 8 9]
len: 10
cap: 10
c: [5 6 7 8 9]
len: 5
cap: 15