When casting the Alarm spell, does the caster need to explicitly specify which creatures to ignore?

Question

In the D&D 5e PHB, on page 211 there is a 1st level spell called Alarm. In the description, it has a sentence:

When you cast the spell, you can designate creatures that won't set off the alarm.

This leaves it a little vague as to whether or not said creatures must be explicitly stated, or whether general categories of creatures can be indicated. It makes sense to explicitly name party members, with the exception of the rogue, that will not set off the alarm. However, what about designating creatures by groups? Here are several examples:

• The city guards won't set off the alarm (some of which the spell caster knows and others they do not know)
• Tiny creatures won't set off the alarm
• Tiny insects won't set off the alarm (telling if something is tiny is not too bad, but how would the spell know if the creature is an insect)
• Good aligned creatures won't sent off the alarm

Are there any clarifications or restrictions on how one can designate which creatures the Alarm spell ignores?

Answer 1

Ask your GM how specific they will let the spell be

The spell only states:

[...] Until the spell ends, an alarm alerts you whenever a tiny or larger creature touches or enters the warded area. When you cast the spell you can designate creatures that won't set off the alarm...

It never further specifies how you can designate the creatures that will not trigger the alarm, how specific you can be, or anything of the sort. This leaves it up to your GM to decide.

Personally I would look to other spells and what wording they use:
The locate creature spell states:

Describe or name a creature that is familiar to you...
The spell can locate a specific creature known to you, or the nearest creature of a specific kind...

The scrying spell states:

You can see and hear a particular creature...

The message spell states:

You point your finger toward a creature within range and Whisper a message...

The magic circle spell states:

[...] Choose one or more of the following types of creatures - Celestials, Elementals, fey, Fiends, or Undead...

The magic mouth spell states:

[...] The triggering circumstance can be as general or as detailed as you like...

Using these we can see that alarm does not require a specific creature, nor even a familiar creature. It also doesn't require a creature to be within some distance. However, it does not say, like magic mouth does, that the trigger can be as specific as you want. This leaves it up to the GM to determine what counts as "too specific".

Answer 2

No not with RAW

As pointed out in your question, there are not a specific parameters for which creatures to designate.

The alarm spell description states (emphasis mine):

When you cast the spell, you can designate creatures that won't set off the alarm.

This can perhaps be an issue as there are very few limitations on who to designate.

Possible workaround

Due to this unspecific nature, one solution would be to rule that when alarm is cast, any creature within the range (30 feet which is the spell range) of the spell can be designated not to set off the alarm.

Meanwhile, any creature within range at the time of casting, but which have not been designated by the caster, can still trigger the alarm by meeting its condition (e.g. opening a door).

By this approach, you also ensures that there is a physical limit to how many creatures which can be designated, if this is an issue.