diff --git a/src/fundamental/bootspec-fundamental.c b/src/fundamental/bootspec-fundamental.c
index 51078397b6e..9c4aee9744e 100644
--- a/src/fundamental/bootspec-fundamental.c
+++ b/src/fundamental/bootspec-fundamental.c
@@ -44,7 +44,7 @@ sd_bool bootspec_pick_name_version(
         good_version = os_image_version ?: (os_version ?: (os_version_id ? : os_build_id));
 
         if (!good_name || !good_version)
-                return false;
+                return sd_false;
 
         if (ret_name)
                 *ret_name = good_name;
@@ -52,5 +52,5 @@ sd_bool bootspec_pick_name_version(
         if (ret_version)
                 *ret_version = good_version;
 
-        return true;
+        return sd_true;
 }
diff --git a/src/fundamental/macro-fundamental.h b/src/fundamental/macro-fundamental.h
index c52957a55c3..cd9e60cf60e 100644
--- a/src/fundamental/macro-fundamental.h
+++ b/src/fundamental/macro-fundamental.h
@@ -85,8 +85,8 @@
 #define ONCE __ONCE(UNIQ_T(_once_, UNIQ))
 #define __ONCE(o)                                                       \
         ({                                                              \
-                static bool (o) = false;                                \
-                __sync_bool_compare_and_swap(&(o), false, true);        \
+                static bool (o) = sd_false;                             \
+                __sync_bool_compare_and_swap(&(o), sd_false, sd_true);  \
         })
 
 #undef MAX
@@ -236,7 +236,7 @@
 
 #define IN_SET(x, ...)                          \
         ({                                      \
-                sd_bool _found = false;         \
+                sd_bool _found = sd_false;      \
                 /* If the build breaks in the line below, you need to extend the case macros. (We use "long double" as  \
                  * type for the array, in the hope that checkers such as ubsan don't complain that the initializers for \
                  * the array are not representable by the base type. Ideally we'd use typeof(x) as base type, but that  \
@@ -245,7 +245,7 @@
                 assert_cc(ELEMENTSOF(__assert_in_set) <= 20); \
                 switch(x) {                     \
                 FOR_EACH_MAKE_CASE(__VA_ARGS__) \
-                        _found = true;          \
+                        _found = sd_true;       \
                         break;                  \
                 default:                        \
                         break;                  \
diff --git a/src/fundamental/types-fundamental.h b/src/fundamental/types-fundamental.h
index 2a9a114bbc0..5977e40c6cb 100644
--- a/src/fundamental/types-fundamental.h
+++ b/src/fundamental/types-fundamental.h
@@ -1,6 +1,20 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+/* This defines a number of basic types that are one thing in userspace and another in the UEFI environment,
+ * but mostly the same in concept and behaviour.
+ *
+ * Note: if the definition of these types/values has slightly different semantics in userspace and in the
+ * UEFI environment then please prefix its name with "sd_" to make clear these types have special semantics,
+ * and *we* defined them. Otherwise, if the types are effectively 100% identical in behaviour in userspace
+ * and UEFI environment you can omit the prefix. (Examples: sd_char is 8 bit in userspace and 16 bit in UEFI
+ * space hence it should have the sd_ prefix; but size_t in userspace and UINTN in UEFI environment are 100%
+ * defined the same way ultimately, hence it's OK to just define size_t as alias to UINTN in UEFI
+ * environment, so that size_t can be used everywhere, without any "sd_" prefix.)
+ *
+ * Note: we generally prefer the userspace names of types and concepts. i.e. if in doubt please name types
+ * after the userspace vocabulary, and let's keep UEFI vocabulary specific to the UEFI build environment. */
+
 #ifdef SD_BOOT
 #include <efi.h>
 
@@ -9,8 +23,8 @@ typedef CHAR16  sd_char;
 typedef INTN    sd_int;
 typedef UINTN   size_t;
 
-#define true    TRUE
-#define false   FALSE
+#define sd_true  TRUE
+#define sd_false FALSE
 #else
 #include <stdbool.h>
 #include <stdint.h>
@@ -18,4 +32,8 @@ typedef UINTN   size_t;
 typedef bool    sd_bool;
 typedef char    sd_char;
 typedef int     sd_int;
+
+#define sd_true  true
+#define sd_false false
+
 #endif