One of the current trends in the development of the mobile application market is the development and adaptation of mobile applications that perform the functions of expert systems. A mobile app is software designed to work on tablets, smartphones, and other mobile devices. An expert system is a set of components that allow the system to make decisions at the level of a competent specialist (expert). The paper considers the concept of building knowledge base models based on a hierarchical functional system and its use in online consultation for mobile expert systems. The article analyzes the structure of knowledge base models in information and communication technologies for choosing: a mobile operator package, a provider, a hoster, and an online store. The model of the domain knowledge base in the “KARKAS” system consists of a hierarchy of domain classes, relations between them (rules) that operate within this model. The “KARKAS” system is a tool for developing knowledge base models for expert systems. To represent knowledge, a hierarchical functional system is developed, which is generated by the system based on the rules of products and frames. The output machine uses a hierarchical functional system during user consultation. The following modes of operation of the output machine are possible: direct output, reverse output, indirect output, Bayes formulas, criteria table, when the product sequence is a list of parameters. The system is implemented using the Embarcadero Delphi 10.4 platform. With the advent of the smartphone, the concept of an intelligent interface for messaging with a mobile application has increased in popularity. The TELEGRAM messenger chatbots are integrated into the “KARKAS” system: @Ribs_karkas_bot, @es_test_karkas_bot, @es_economy_karkas_bot, @es_info_tech_karkas_bot, which allow you to conduct online consultation, exchange messages between knowledge base models and chatbots. Embarcadero's cross-platform FireMonkey (FMX) framework allows you to quickly create application prototypes for running on different platforms: Windows, Android, and iOS. The purpose of the research is to develop and justify a fully functional prototype of the “KARKAS” tool system with a user interface in the form of a mobile application for the Android and iOS platforms. The problem of identifying temporal knowledge is urgent in solving many problems in the field of artificial intelligence. There are several ways to solve it, for example, the traditional direction is the explicit use of time in temporal knowledge models. Another approach is the use of time implicit in the idea of separation of the knowledge base. Functional system ‒ this is a system formed to achieve a given useful result (objective function) in the course of its functioning. Its system-forming factor is a specific result. In other words, the goal is considered as a given result, and the constraints are considered as the degree of freedom necessary to achieve the result. For example, a functional system can be considered as a set of functions with a certain set of operations applied to these functions. The role of functions is played by the rules of the knowledge base, and the main operations are the mapping of an attribute to a sample and determining the conditions for the applicability of the rules. The functional system is characterized by the following properties: connectivity-a chain of bundles of the knowledge base; complexity-hierarchy of levels of local knowledge bases; stability (adaptive behavior of the system) – the structure of the digraph of the functional system does not change with vertical perturbations of the rules. In other words, only the values of the antecedents of the rules for the local knowledge bases of the bundle chain are changed, and the bundle base, which is interpreted as the external environment, remains unchanged. In this paper, the knowledge base model is considered as a hierarchical functional system in which the result has an organizing influence on all stages of ontology formation. Classes and relationships between them can be considered as a logical construction of a functional system.