Only cross-shard native token (ONE token) transfers are allowed. No cross-shard for HRC20 or other contracts. Meaning, smart contracts are deployed on shard-0 and all contract interactions happen on shard-0. Contracts can still be deployed on other shards, however they won't be able to interact with contracts in other shards. We have cross-shard smart contract on our roadmap for Q3, 2021.