Application prototypes support LLD macros.
One application prototype can be used by several item prototypes of the same discovery rule.
If created application prototype is not used by any item prototype it gets removed from 'Application prototypes' list automatically.
Like other discovered entities applications follow the lifetime defined in discovery rule ('keep lost resources period' setting) - they are removed after not being discovered for the specified number of days.
If an application is not discovered anymore, the application itself may not be removed because of the 'lost resources period' setting, however:
Application prototypes defined by one discovery rule can't discover the same application. In this situation only the first prototype discovery will succeed, the rest will report appropriate LLD error. Only application prototypes defined in different discovery rules can result in discovering the same application.
LLD macros may be used inside user macro context, for example, in trigger prototypes.
Since Zabbix agent version 3.2 it is possible to define several low-level discovery rules with the same discovery item.
To do that you need to define the Alias agent parameter, allowing to use altered discovery item keys in different discovery rules, for example
There is no limit for low-level discovery rule JSON data if it is received directly by Zabbix server, because return values are processed without being stored in a database. There's also no limit for custom low-level discovery rules, however, if it is intended to acquire custom LLD data using a user parameter, then the user parameter return value limit applies (512 KB).
If data has to go through Zabbix proxy it has to store this data in database so database limits apply.