问题：
I have a question about Emacs Lisp.我有一个关于 Emacs Lisp 的问题。 What is the difference between setq and setq-default ? setq和setq-default 有什么区别？
Tutorials say setq takes effect in the local buffer while setq-default affects all buffers.教程说setq在本地缓冲区中生效，而setq-default影响所有缓冲区。
For example, if I wrote (setq a-var a-vars-value) in init.el , I found after starting Emacs and opening a new buffer, the a-var is also there and its value is a-vars-value .例如，如果我在init.el 中写了(setq a-var a-vars-value) ，我发现在启动 Emacs 并打开一个新缓冲区后， a-var也在那里，它的值为a-vars-value 。 I thought it was not supposed to be there.我认为它不应该在那里。 It seems there is no difference between setq and setq-default .似乎setq和setq-default之间没有区别。
Is there something wrong with my understanding?我的理解有问题吗？
For example:例如：
I wrote (setq hello 123) in the init.el file, and I run emacs abuffer in the shell, then I input "hello Cx Ce" , it shows "123".我在init.el文件中写了(setq hello 123) ，在shell中运行emacs abuffer ，然后输入“hello Cx Ce” ，显示“123”。 The same happens when I run this in all new buffers.当我在所有新缓冲区中运行它时，也会发生同样的情况。
I wrote (setq tab-width 4) in the init.el file.我在init.el文件中写了(setq tab-width 4) 。 When I run tab-width Cx Ce , it shows "8" (Current mode is 'Text').当我运行tab-width Cx Ce 时，它显示“8”（当前模式为“文本”）。 However, when I use (setq-default tab-width 4) , it show "4".但是，当我使用(setq-default tab-width 4) 时，它显示“4”。 I can't explain this phenomenon.我无法解释这种现象。