Original text:
In order to help clients configure themselves with the right URIs for each ACME operation, ACME servers provide a directory object.  This should be the only URL needed to configure clients.  It is a JSON <del> dictionary, whose keys are drawn from the following table and whose </del>  values are the corresponding URLs. There is no constraint on the actual URI of the directory except that a host which only functions as an ACME server could place the directory under path "/". <del> The dictionary MAY additionally contain a key "meta".  If present, it MUST be a JSON dictionary; each item in the dictionary is an item of </del>  metadata relating to the service provided by the ACME server. The following metadata items are defined, all of which are OPTIONAL: The challenges that the client can fulfill in order to prove possession of the identifier (for pending authorizations).  For final authorizations, the challenges that were used.  Each array <del> entry is a dictionary with parameters required to validate the </del>  challenge.  A client should attempt to fulfill at most one of these challenges, and a server should consider any one of the challenges sufficient to make the authorization valid. client needs to respond with information to complete the challenges. To do this, the client updates the authorization object received from the server by filling in any required information in the elements of <del> the "challenges" dictionary.  (This is also the stage where the client should perform any actions required by the challenge.) </del>  The client sends these updates back to the server in the form of a JSON object with the response fields required by the challenge type, Template: <del> Field name: The string to be used as a key in the JSON dictionary </del>  Field type: The type of value to be provided, e.g., string, boolean, array of string Template: <del> Key: The value to be used as a dictionary key in the directory object </del>  Resource type: The type of resource labeled by the key

Reviewer comments:
Follow-up to , catches a few more references I initially missed.
I think 'field name' makes more sense... but I'm not really wed to either.

Revised text:
In order to help clients configure themselves with the right URIs for each ACME operation, ACME servers provide a directory object.  This should be the only URL needed to configure clients.  It is a JSON  <ins> object, whose keys are drawn from the following table and whose </ins> values are the corresponding URLs. There is no constraint on the actual URI of the directory except that a host which only functions as an ACME server could place the directory under path "/".  <ins> The object MAY additionally contain a key "meta".  If present, it MUST be a JSON object; each field in the object is an item of </ins> metadata relating to the service provided by the ACME server. The following metadata items are defined, all of which are OPTIONAL: The challenges that the client can fulfill in order to prove possession of the identifier (for pending authorizations).  For final authorizations, the challenges that were used.  Each array  <ins> entry is an object with parameters required to validate the </ins> challenge.  A client should attempt to fulfill at most one of these challenges, and a server should consider any one of the challenges sufficient to make the authorization valid. client needs to respond with information to complete the challenges. To do this, the client updates the authorization object received from the server by filling in any required information in the elements of  <ins> the "challenges" object.  (This is also the stage where the client should perform any actions required by the challenge.) </ins> The client sends these updates back to the server in the form of a JSON object with the response fields required by the challenge type, Template:  <ins> Field name: The string to be used as a key in the JSON object </ins> Field type: The type of value to be provided, e.g., string, boolean, array of string Template:  <ins> Key: The value to be used as a field name in the directory object </ins> Resource type: The type of resource labeled by the key