How do subscriber tags work?

Tags are applied to subscribers if they, at any time, send a message to your campaign that matches a keyword you’ve specified in a previous broadcast.

To learn more about how tags and keywords work to send segmented broadcasts, please check our article: How do I send a segmented broadcast? This article will go over some of the important nuances about how to create working tags and how they get applied to users.
First, let's identify what terms/characters can be used to create a working tag. 
Subscriber tags can contain any amount of alphanumeric characters, and multi-word tags can contain spaces, hyphens, and underscores. Subscriber tags can not contain any other special characters or punctuation, protected SMS keywords, or emojis. Refer to the following table for examples.
Keywords
Classification
Allowed?
coffee, tea, weekend, go, new york city, san francisco 49ers, 212, under 35
Alphanumeric keywords
Allowed. Tags can be any length and consist of any combination of letters, numbers, and spaces.
stop, stopall, unsubscribe, cancel, end, quit, start, yes, unstop, help, info, agree
SMS Keywords
Not allowed. These words are protected SMS keywords.
mother-in-law, 10-4, cool_cats
Valid special characters
Allowed. Tags can contain dashes and underscores.
100%, yes! , “quote” , :) , website.com , #awesome
Invalid special characters
Not allowed. Tags can not contain any other special characters.
grinning face, american football, i red heart u, pizza pizza 
Emojis
Not allowed. Tags can not contain emojis.
 
For a tag to be successfully applied to a user, they must text back ONLY the tagged word.  Consider the following example: You’ve created a broadcast encouraging your subscribers to text back either boston or new york city to receive tailored updates pertaining to their city of choice. Refer to the following table to see which responses result in successful tagging.
 
Subscriber Reply
Tag Applied Successfully?
Reason
boston
Yes, the boston tag is applied.
Reply matches the tag exactly.
BOSTON
Yes, the boston tag is applied.
Tags are not case sensitive.
i love boston
No, the boston tag is not applied.
Replies must consist of only the specified tag in order for the tag to be applied.
boston (several weeks after you sent the initial broadcast, and you’ve sent multiple broadcasts since then) 
Yes, the boston tag is applied.
Once a subscriber tag is created, a subscriber can opt into it at any time.
new york city
Yes, the new york city tag is applied.
Reply matches the tag exactly.
new york
No, the new york city tag is not applied.
Tags must be typed in their entirety to be applied to a subscriber.
new york city in one message and boston in another message
Yes, both tags are applied.
There is no limit to how many tags a subscriber can have applied to them, as long as they reply with the tags in separate messages.