Questions regarding pbtxt format

I am trying to read a .pbtxt file and have two questions regarding the following section:

node {
  name: "bert/embeddings/Slice/begin"
  op: "Const"
  input: "^bert/embeddings/assert_less_equal/Assert/Assert"
  attr {
    key: "value"
    value {
      tensor {
        dtype: DT_INT32
        tensor_shape {
          dim {
            size: 2
        tensor_content: "\000\000\000\000\000\000\000\000"
  1. Why does a Const node have an input? Is the value not defined by the attr part of this node?
  2. What does the ^ mean at the beginning of the input field?
  3. Is there documentation available for the TensorFlow pbtxt format? I was not able to find anything.

Many thanks for your help in advance.