In a Flutter project, package dependencies are defined in the pubspec.yaml file, which is located at the root of the project. This file includes information about the packages used in the project, such as the package name and the required version.

For example, a fragment of the pubspec.yaml file might look like this:

dependencies:
  flutter:
    sdk: flutter
  cupertino_icons: ^1.0.2
  http: ^0.13.3

In this example, the project depends on the latest version of the Flutter SDK, as well as two additional packages: cupertino_icons and http. The syntax ^1.0.2 indicates that a version of cupertino_icons is required that is equal to or greater than 1.0.2, but less than 2.0.0. Similarly, ^0.13.3 indicates that an http version is required that is equal to or greater than 0.13.3, but less than 0.14.0.

When the flutter packages get command is executed, Flutter reads the pubspec.yaml file and downloads the specified dependent packages. If this command is run multiple times, the Flutter package manager will attempt to download the latest version of each package, which may result in different versions of the packages being installed at different times.

To ensure that the same versions of packages are used each time the flutter packages get command is run, Flutter creates a file called pubspec.lock. This file includes a list of the package dependencies used in the project, as well as the specific versions of those packages that were used at the time flutter packages get was run.

For example, a snippet of the pubspec.lock file might look like this:

http:
  dependency: "direct main"
  description:
    name: http
    url: "https://pub.dartlang.org"
  source: hosted
  version: "0.13.3"

In this example, you can see that version 0.13.3 of the http package was used at the time flutter packages get was run.

The function of the pubspec.lock file is to ensure that each time flutter packages get to run on the project, the same versions of the dependent packages are installed. This is important because different versions of a package may have different behaviour and functionality, and there may be incompatibilities between versions of different packages.

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It is recommended to include the pubspec.lock file only in application packages and not in library packages because library packages are often used by multiple projects and adding the pubspec.lock file in a library package can create conflicts.

Library packages are packages that contain code that can be used by multiple projects, while application packages are packages that contain application-specific code. When a library package is published to the Flutter package repository, it includes a pubspec.lock file that was generated by running flutter packages gets when creating the package version.

If a library package includes the pubspec.lock file, it can cause conflicts in projects that use it. For example, if two projects use the same library package, but in different versions, and both include the pubspec.lock file generated by the library package, there may be version conflicts that can cause one or both projects to not work correctly.