我已经搜索过谷歌，但到目前为止我唯一能找到的是以下内容引用自苹果开发者论坛 https://forums.developer.apple.com/message/9176#9176:
对于那些好奇的人来说，.tbd 文件是新的“基于文本的存根”
库”，提供更紧凑的存根版本
在 SDK 中使用的库，并有助于显着减少
下载大小。
希望更多的文档很快就会出现。
Update
作为示例，以下是完整内容libsqlite3.tbd。它只是一个文本文件。请注意，安装名称是libsqlite3.dylib.
---
archs: [ armv7, armv7s, arm64 ]
platform: ios
install-name: /usr/lib/libsqlite3.dylib
current-version: 216.4
compatibility-version: 9.0
exports:
- archs: [ armv7, armv7s, arm64 ]
symbols: [ __sqlite3_lockstate, __sqlite3_purgeEligiblePagerCacheMemory,
__sqlite3_system_busy_handler, __sqlite_auto_profile,
__sqlite_auto_profile_syslog, __sqlite_auto_trace,
__sqlite_auto_trace_syslog, _sqlite3OsShmHasMultipleLinks,
_sqlite3OsShmRenamedWhileOpen, _sqlite3OsShmWasTruncated,
_sqlite3OsShmWasUnlinkedWhileOpen, _sqlite3VersionNumber,
_sqlite3VersionString, _sqlite3_aggregate_context,
_sqlite3_aggregate_count, _sqlite3_auto_extension,
_sqlite3_backup_finish, _sqlite3_backup_init, _sqlite3_backup_pagecount,
_sqlite3_backup_remaining, _sqlite3_backup_step,
_sqlite3_bind_blob, _sqlite3_bind_blob64, _sqlite3_bind_double,
_sqlite3_bind_int, _sqlite3_bind_int64, _sqlite3_bind_null,
_sqlite3_bind_parameter_count, _sqlite3_bind_parameter_index,
_sqlite3_bind_parameter_name, _sqlite3_bind_text,
_sqlite3_bind_text16, _sqlite3_bind_text64, _sqlite3_bind_value,
_sqlite3_bind_zeroblob, _sqlite3_blob_bytes, _sqlite3_blob_close,
_sqlite3_blob_open, _sqlite3_blob_read, _sqlite3_blob_reopen,
_sqlite3_blob_write, _sqlite3_busy_handler, _sqlite3_busy_timeout,
_sqlite3_cancel_auto_extension, _sqlite3_changes,
_sqlite3_clear_bindings, _sqlite3_close, _sqlite3_close_v2,
_sqlite3_collation_needed, _sqlite3_collation_needed16,
_sqlite3_column_blob, _sqlite3_column_bytes, _sqlite3_column_bytes16,
_sqlite3_column_count, _sqlite3_column_decltype,
_sqlite3_column_decltype16, _sqlite3_column_double,
_sqlite3_column_int, _sqlite3_column_int64, _sqlite3_column_name,
_sqlite3_column_name16, _sqlite3_column_text, _sqlite3_column_text16,
_sqlite3_column_type, _sqlite3_column_value, _sqlite3_commit_hook,
_sqlite3_compileoption_get, _sqlite3_compileoption_used,
_sqlite3_complete, _sqlite3_complete16, _sqlite3_config,
_sqlite3_context_db_handle, _sqlite3_create_collation,
_sqlite3_create_collation16, _sqlite3_create_collation_v2,
_sqlite3_create_function, _sqlite3_create_function16,
_sqlite3_create_function_v2, _sqlite3_create_module,
_sqlite3_create_module_v2, _sqlite3_data_count,
_sqlite3_data_directory, _sqlite3_db_config, _sqlite3_db_filename,
_sqlite3_db_handle, _sqlite3_db_mutex, _sqlite3_db_readonly,
_sqlite3_db_release_memory, _sqlite3_db_status,
_sqlite3_declare_vtab, _sqlite3_enable_shared_cache,
_sqlite3_errcode, _sqlite3_errmsg, _sqlite3_errmsg16,
_sqlite3_errstr, _sqlite3_exec, _sqlite3_expired,
_sqlite3_extended_errcode, _sqlite3_extended_result_codes,
_sqlite3_file_control, _sqlite3_finalize, _sqlite3_free,
_sqlite3_free_table, _sqlite3_get_autocommit, _sqlite3_get_auxdata,
_sqlite3_get_table, _sqlite3_global_recover, _sqlite3_initialize,
_sqlite3_intarray_bind, _sqlite3_intarray_create,
_sqlite3_interrupt, _sqlite3_last_insert_rowid,
_sqlite3_libversion, _sqlite3_libversion_number,
_sqlite3_limit, _sqlite3_log, _sqlite3_malloc, _sqlite3_malloc64,
_sqlite3_memory_alarm, _sqlite3_memory_highwater,
_sqlite3_memory_used, _sqlite3_mprintf, _sqlite3_msize,
_sqlite3_mutex_alloc, _sqlite3_mutex_enter, _sqlite3_mutex_free,
_sqlite3_mutex_leave, _sqlite3_mutex_try, _sqlite3_next_stmt,
_sqlite3_open, _sqlite3_open16, _sqlite3_open_v2,
_sqlite3_os_end, _sqlite3_os_init, _sqlite3_overload_function,
_sqlite3_prepare, _sqlite3_prepare16, _sqlite3_prepare16_v2,
_sqlite3_prepare_v2, _sqlite3_profile, _sqlite3_progress_handler,
_sqlite3_randomness, _sqlite3_realloc, _sqlite3_realloc64,
_sqlite3_release_memory, _sqlite3_reset, _sqlite3_reset_auto_extension,
_sqlite3_result_blob, _sqlite3_result_blob64, _sqlite3_result_double,
_sqlite3_result_error, _sqlite3_result_error16,
_sqlite3_result_error_code, _sqlite3_result_error_nomem,
_sqlite3_result_error_toobig, _sqlite3_result_int,
_sqlite3_result_int64, _sqlite3_result_null, _sqlite3_result_text,
_sqlite3_result_text16, _sqlite3_result_text16be,
_sqlite3_result_text16le, _sqlite3_result_text64,
_sqlite3_result_value, _sqlite3_result_zeroblob,
_sqlite3_rollback_hook, _sqlite3_rtree_geometry_callback,
_sqlite3_rtree_query_callback, _sqlite3_set_authorizer,
_sqlite3_set_auxdata, _sqlite3_shutdown, _sqlite3_sleep,
_sqlite3_snprintf, _sqlite3_soft_heap_limit, _sqlite3_soft_heap_limit64,
_sqlite3_sourceid, _sqlite3_sql, _sqlite3_status,
_sqlite3_status64, _sqlite3_step, _sqlite3_stmt_busy,
_sqlite3_stmt_readonly, _sqlite3_stmt_status, _sqlite3_strglob,
_sqlite3_stricmp, _sqlite3_strnicmp, _sqlite3_table_column_metadata,
_sqlite3_temp_directory, _sqlite3_test_control,
_sqlite3_thread_cleanup, _sqlite3_threadsafe, _sqlite3_total_changes,
_sqlite3_trace, _sqlite3_transfer_bindings, _sqlite3_update_hook,
_sqlite3_uri_boolean, _sqlite3_uri_int64, _sqlite3_uri_parameter,
_sqlite3_user_data, _sqlite3_value_blob, _sqlite3_value_bytes,
_sqlite3_value_bytes16, _sqlite3_value_double, _sqlite3_value_int,
_sqlite3_value_int64, _sqlite3_value_numeric_type,
_sqlite3_value_text, _sqlite3_value_text16, _sqlite3_value_text16be,
_sqlite3_value_text16le, _sqlite3_value_type, _sqlite3_version,
_sqlite3_vfs_find, _sqlite3_vfs_register, _sqlite3_vfs_unregister,
_sqlite3_vmprintf, _sqlite3_vsnprintf, _sqlite3_vtab_config,
_sqlite3_vtab_on_conflict, _sqlite3_wal_autocheckpoint,
_sqlite3_wal_checkpoint, _sqlite3_wal_checkpoint_v2,
_sqlite3_wal_hook ]
...
我在中找到了这个和其他 .tbd 文件
Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/lib/
如果您转到 Xcode 项目的“常规”选项卡，然后在“链接的框架和库”下添加一个库，您还可以看到一个 .tbd 文件。 .tbd 文件将被复制到您的项目中。
因此，.dylib 文件似乎是您的项目正在使用的实际二进制代码库，位于用户设备上的 /usr/lib/ 目录中。另一方面，.tbd 文件只是一个包含在项目中的文本文件，用作所需 .dylib 二进制文件的链接。由于该文本文件比二进制库小得多，因此使得 SDK 的下载大小更小。
目前我只是根据所提供的信息进行推测，所以如果我错了，请纠正我。
