You will be able to retrieve services from your maps, this could allow you to query objects from the database or from external APIs for example.

public class MyMaps :
	INewMap<Product, ProductDto>,
	IMergeMap<Category, CategoryDto>
{
	ProductDto? INewMap<Product, ProductDto>.Map(Product? source, MappingContext context){
		if(source == null)
			return null;
		else{
			var product = context.ServiceProvider.GetRequiredService<MyDatabase>().Find<Product>(source.Code);

			return new ProductDto{
				Code = source.Code,
				Name = product.Name,
				...
			};
		}
	}

	CategoryDto? IMergeMap<Category, CategoryDto>.Map(Category? source, CategoryDto? destination, MappingContext context){
		if(source != null){
			var category = context.ServiceProvider.GetRequiredService<MyAPI>().GetCategoryParent(source.Id);

			destination ??= new CategoryDto();
			destination.Id = source.Id;
			destination.Parent = category?.Id;
			...
		}
		return destination;
	}
}