diff --git a/src/pb/sample.rs b/src/pb/sample.rs
index 54dea90..6945105 100644
--- a/src/pb/sample.rs
+++ b/src/pb/sample.rs
@@ -6,6 +6,16 @@ use super::Publisher;
 
 use std::ops::{Deref, DerefMut};
 
+/// # Safety
+///
+/// This is a marker trait for types that can be transferred via shared memory.
+/// The types must satisfy the following conditions:
+/// - no heap is used
+/// - the data structure is entirely contained in the shared memory - no pointers
+///   to process local memory, no references to process local constructs, no dynamic allocators
+/// - the data structure has to be relocatable and therefore must not internally
+///   use pointers/references
+/// In general, types that could implement the Copy trait fulfill these requirements.
 pub unsafe trait POD {}
 // TODO more impls
 unsafe impl POD for i8 {}