Unreal's USTRUCT Specifiers page lists all of the core specifiers but a lot of the metadata specifiers that it lists are only usable with UCLASS.

This page attempts to be an exhaustive list of all the USTRUCT specifiers, giving explanations, sample code, screenshots and related links for each.

General Points

  • Tags are case insensitive, but I would recommend sticking to the case example here for readability.
  • Quotation marks are optional if the values does not have spaces. e.g. someBool=true is treated the same as someBool="true". Again, for readability I would recommend using quotation marks only around string-type properties.

Blueprint Logic

Blueprint Type

Position:

Main

Type:

flag

USTRUCT(BlueprintType)

Setting this allows the struct to be added as a variable to Blueprints. To expose a struct to blueprints with BlueprintReadOnly, the struct must be marked with BlueprintType.

Exposes this struct as a type that can be used for variables in Blueprints.

Unreal Documentation

Hidden By Default

Position:

Meta

Type:

flag

USTRUCT(meta=(HiddenByDefault))

Pins in Make and Break nodes are hidden by default.

Unreal Documentation

Disable Split Pin

Position:

Meta

Type:

flag

Related:

USTRUCT(meta=(DisableSplitPin))

"Split Struct Pin" is an option available when right-clicking the return node of a UFUNCTION. It is only available if all the variables within the struct are exposed to blueprints, and if DisableSplitPin is not present in the USTRUCT definition.

Why you might want to use it is a more interesting question. The only example of this being used in the codebase is with FGameplayTag. Gameplay tags are structs but only contain an FName property that is used as their ID. I suppose that DisableSplitPin is used because Epic wanted to discourage users from splitting the struct out to FName when it's much more helpful to keep it as an FStruct and use all the helper functions within.

Indicates that node pins of this struct type cannot be split

Unreal Documentation 
USTRUCT(meta=(DisableSplitPin))
struct FGameplayTag
{
    GENERATED_BODY()
    UPROPERTY(VisibleAnywhere, Category = GameplayTags)
    FName TagName;
};

Has Native Break

Position:

Meta

Type:

string (Function name)

Related:

USTRUCT(meta=(HasNativeBreak="abc"))

HasNativeBreak lets you specify a static UFUNCTION() to use instead of the default behavior when a user chooses "Break (Struct)"

By default all properties marked as BlueprintReadOnly or BlueprintReadWrite would be present in the node. Defining a native break function can let you customize what is present in that node.

Indicates that the struct has a custom break node (and what the path to the BlueprintCallable UFunction is) that should be used instead of the default BreakStruct node.

Unreal Documentation 
USTRUCT(BlueprintType, meta = (HasNativeBreak = "ExampleProject.CatHelperLibrary.BreakCatStruct"))
struct FCat
{
  GENERATED_BODY()
public:
  UPROPERTY()
  FString Name;
  UPROPERTY()
  FLinearColor CoatColor;
  UPROPERTY()
  FString SecretPetName;
};

UCLASS()
class UCatHelperLibrary : public UBlueprintFunctionLibrary
{
  GENERATED_BODY()
public:
  UFUNCTION(BlueprintPure, Category = "Cat", meta = (BlueprintThreadSafe))
  static void BreakCatStruct(const FCat& Cat, FString& OutName, FLinearColor& OutCoatColor);
};
void UCatHelperLibrary::BreakCatStruct(const FCat& Cat, FString& OutName, FLinearColor& OutCoatColor)
{
  OutName = Cat.Name;
  OutCoatColor = Cat.CoatColor;
}

Has Native Make

Position:

Meta

Type:

string (Function name)

Related:

USTRUCT(meta=(HasNativeMake="abc"))

Similar to HasNativeBreak, HasNativeMake allows you to specify a static UFUNCTION() to use to create and fill the values of a struct, without having to make the values within the struct BlueprintReadWrite.

Indicates that the struct has a custom make node (and what the path to the BlueprintCallable UFunction is) that should be used instead of the default MakeStruct node.

Unreal Documentation 
USTRUCT(BlueprintType, meta = (HasNativeMake = "ExampleProject.CatHelperLibrary.MakeCatStruct"))
struct FCat
{
  GENERATED_BODY()
public:
  UPROPERTY()
  FString Name;
  UPROPERTY()
  FLinearColor CoatColor;
  UPROPERTY()
  FString SecretPetName;
};

UCLASS()
class UCatHelperLibrary : public UBlueprintFunctionLibrary
{
  GENERATED_BODY()
public:
  UFUNCTION(BlueprintPure, Category = "Cat", meta = (BlueprintThreadSafe))
  static FCat MakeCatStruct(FString Name, FLinearColor CoatColor);
};

FCat UCatHelperLibrary::MakeCatStruct(FString Name, FLinearColor CoatColor)
{
  FCat Cat;
  Cat.Name = Name;
  Cat.CoatColor = CoatColor;
  return Cat;
}

C++

No Export

Position:

Main

Type:

flag

USTRUCT(NoExport)

"No autogenerated code will be created" means you don't need to write GENERATED_BODY

No autogenerated code will be created for this class; the header is only provided for parsing metadata.

Unreal Documentation

Deprecated

Immutable

Position:

Main

Type:

flag

USTRUCT(Immutable)

Immutable is only legal in Object.h and is being phased out, do not use on new structs!.

Unreal Documentation

Atomic

Position:

Main

Type:

flag

USTRUCT(Atomic)

This is listed in the official documentation but I can't find a single use of it inside the Unreal Engine codebase.

Indicates that this struct should always be serialized as a single unit. No autogenerated code will be created for this class; the header is only provided to parse metadata from.

Unreal Documentation