I think the way Crossplane explains this could be clearer.

CompositeResourceDefinition (XRD, related but not exactly the same as a CRD) – this is like the interface for a function that defines the parameters, their types and which are required.

apiVersion: apiextensions.crossplane.io/v1
kind: CompositeResourceDefinition
metadata:
  name: xglobal-vault-accesses.azure.simbachain.com
spec:
  group: azure.simbachain.com
  # for the CompositeResource XR, the actual resource created by the claim
  names:
    kind: XGlobal-Vault-Access
    plural: xglobal-vault-accesses
  claimNames:
    kind: Global-Vault-Access
    plural: global-vault-accesses
  versions:
    - name: v1beta1
      served: true
      referenceable: true
      schema:
        openAPIV3Schema:
<<snip>>

Composition – this is like the internal implementation code of the function, note however that there can be more than one implementation of a given function with labels used to distinguish them.

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
  name: global-vault-access
  labels:
    crossplane.io/xrd: xglobal-vault-accesses.azure.simbachain.com
    provider: azure
spec:
  compositeTypeRef:
    apiVersion: azure.simbachain.com/v1beta1
    kind: XGlobal-Vault-Access
  resources:
<<snip>>

Claim – this is a request like the invocation of the function, you pass in your parameters. Note: the kind is not Claim, it will be whatever was under claimNames > kind. The word Claim generally does not appear anywhere, you can add it to a comment if that helps

# below is a crossplane claim that requests an instance of XGlobal-Vault-Access and its managed resources
---
apiVersion: azure.simbachain.com/v1beta1
kind: Global-Vault-Access
metadata:
  annotations:
    crossplane.io/external-name: blocks-simba-kongtest
  # will be used to generate names for the resources in the composition
  name: blocks-simba-kongtest
  namespace: blocks-simba-kongtest
spec:
  compositionRef:
    name: global-vault-access
  parameters:
    location: eastus
<<snip>>

CompositeResource (XR) – Sometimes this is talked about as if it’s the same as the claim but it’s a separate Kubernetes resource and it’s important to differentiate them. This results from the resources above instead of being defined directly. You define an XRD, make a Composition that defines the internal implementation of that XRD, make a claim that passes parameters then a CompositeResource will be created automatically. When I am troubleshooting this is the first place I want to look because it will has Synced and Ready status and contains resource refs to all the individual resources. Note: the kind is not CompositeResource, it will be whatever was under names > kind. It is traditionally named starting with X in the XRD.

kubectl describe xGlobal-Vault-Access