Is a lookup definition necessary?

[Junie]

I've uploaded a lookup csv file, and was immediately able to use it from a inputlookup and lookup (referencing the file). I'm wondering why a lookup definition is needed to be set per the instructions from the Search tutorial? https://docs.splunk.com/Documentation/Splunk/8.0.1/SearchTutorial/Usefieldlookups

"It is not sufficient to share the lookup table file with an application. You must define the information in the lookup table file and how that information relates to the fields in your events. This is called a lookup definition.

[gcusello]

Hiu @Junie,
as you can read at https://docs.splunk.com/Documentation/Splunk/8.0.1/Knowledge/Aboutlookupsandfieldactions#Lookup_defi... lookup definition is mandatory:

A lookup definition provides a lookup name and a path to find the lookup table. Lookup definitions can include extra settings such as matching rules, or restrictions on the fields that the lookup is allowed to match. One lookup table can have multiple lookup
definitions.
All lookup types require a lookup definition. After you create a lookup definition you can invoke the lookup in a search with the lookup command.

I have seen that without lookup definition the inputlookup command runs without problems, instead the lookup command has problems.

Ciao.
Giuseppe