diff --git a/tuf/api/metadata.py b/tuf/api/metadata.py
index 9cde07e08f..716b585479 100644
--- a/tuf/api/metadata.py
+++ b/tuf/api/metadata.py
@@ -379,6 +379,10 @@ def __init__(
             self, _type: str=None, version: int=None, spec_version: str=None,
             expires: datetime=None, consistent_snapshot: bool=None,
             keys: JsonDict=None, roles: JsonDict=None) -> None:
+
+        # TODO: Add sensible defaults when we have input validation.
+        # See issue https://github.com/theupdateframework/tuf/issues/1140
+        # We need default values to create empty objects in Signed.from_dict()
         super().__init__(_type, version, spec_version, expires)
         # TODO: Add classes for keys and roles
         self.consistent_snapshot = consistent_snapshot
@@ -491,6 +495,10 @@ def __init__(
             self, _type: str=None, version: int=None, spec_version: str=None,
             expires: datetime=None, meta: Dict[str, MetadataInfo]=None
             ) -> None:
+
+        # TODO: Add sensible defaults when we have input validation.
+        # See issue https://github.com/theupdateframework/tuf/issues/1140
+        # We need default values to create empty objects in Signed.from_dict()
         super().__init__(_type, version, spec_version, expires)
         self.meta = meta
 
@@ -525,6 +533,8 @@ def update(self, version: int, length: Optional[int] = None,
             hashes: Optional[JsonDict] = None) -> None:
         """Assigns passed info about snapshot metadata to meta dict. """
 
+        # TODO: Consider renaming this function:
+        # see: https://github.com/theupdateframework/tuf/issues/1230
         self.meta['snapshot.json'] = MetadataInfo(version, length, hashes)
 
 
@@ -546,6 +556,10 @@ def __init__(
             self, _type: str=None, version: int=None, spec_version: str=None,
             expires: datetime=None, meta: Dict[str, MetadataInfo]=None
             ) -> None:
+
+        # TODO: Add sensible defaults when we have input validation.
+        # See issue https://github.com/theupdateframework/tuf/issues/1140
+        # We need default values to create empty objects in Signed.from_dict()
         super().__init__(_type, version, spec_version, expires)
         self.meta = meta
 
@@ -583,6 +597,9 @@ def update(
             self, rolename: str, version: int, length: Optional[int] = None,
             hashes: Optional[JsonDict] = None) -> None:
         """Assigns passed (delegated) targets role info to meta dict. """
+
+        # TODO: Consider renaming this function:
+        # see: https://github.com/theupdateframework/tuf/issues/1230
         metadata_fn = f'{rolename}.json'
 
         self.meta[metadata_fn] = MetadataInfo(version, length, hashes)
@@ -693,6 +710,10 @@ def __init__(
             self, _type: str=None, version: int=None, spec_version: str=None,
             expires: datetime=None, targets: Dict[str, TargetInfo]=None,
             delegations: JsonDict=None) -> None:
+
+        # TODO: Add sensible defaults when we have input validation.
+        # See issue https://github.com/theupdateframework/tuf/issues/1140
+        # We need default values to create empty objects in Signed.from_dict()
         super().__init__(_type, version, spec_version, expires)
         self.targets = targets
 
@@ -732,4 +753,7 @@ def to_dict(self) -> JsonDict:
     # Modification.
     def update(self, filename: str, fileinfo: JsonDict) -> None:
         """Assigns passed target file info to meta dict. """
+
+        # TODO: Consider renaming this function:
+        # see: https://github.com/theupdateframework/tuf/issues/1230
         self.targets[filename] = TargetInfo(**fileinfo)